(Conc. Trid., Sess. xxv. December 3d and 4th, 1563. [Buckley's Trans.])
The holy synod enjoins on all bishops, and others sustaining the office and charge of teaching that, according to the usage of the Catholic and Apostolic Church received from the primitive times of the Christian religion, and according to the consent of the holy Fathers, and to the decrees of sacred councils, they especially instruct the faithful diligently touching the intercession and invocation of saints; the honour paid to relics; and the lawful use of images -- teaching them, that the saints, who reign together with Christ, offer up their own prayers to God for men; that it is good and useful suppliantly to invoke them, and to resort to their prayers, aid and help, for obtaining benefits from God, through his Son, Jesus Christ our Lord, who alone is our Redeemer and Saviour; but that they think impiously, who deny that the saints, who enjoy eternal happiness in heaven, are to be invoked; or to assert either that they do not pray for men; or, that the invocation of them to pray for each of us, even in particular, is idolatry; or, that it is repugnant to the word of God, and is opposed to the honour of the one mediator between God and men, Christ Jesus, or, that it is foolish to supplicate, orally or inwardly, those who reign in heaven. Also, that the holy bodies of holy martyrs and of others now living with Christ, which were the living members of Christ, and the temples of the Holy Ghost, and which are by him to be raised unto eternal life, and to be glorified, are to be venerated by the faithful, through which [bodies] many benefits are bestowed by God on men; so that they who affirm that veneration and honour are not due to the relics of saints; or, that these, and other sacred monuments, are uselessly honoured by the faithful; and that the places dedicated to the memories of the Saints are vainly visited for the purpose of obtaining their aid; are wholly to be condemned, as the Church has already long since condemned, and doth now also condemn them.
Moreover, that the images of Christ, of the Virgin Mother of God and of the other Saints, are to be had and retained particularly in temples, and that due honour and veneration are to be awarded them; not that any divinity or virtue is believed to be in them, on account of which they are to be worshipped; or that anything is to be asked of them; or that confidence is to be reposed in images, as was of old done by Gentiles, who placed their hope in idols; but because the honour which is shown unto them is referred to the prototypes which they represent; in such wise that by the images which we kiss, and before which we uncover the head, and prostrate ourselves, we adore Christ, and venerate the Saints, whose similitude they bear. And this, by the decrees of councils, and especially of the second synod of Nicæa, has been ordained against the opponents of images.
And the bishops shall carefully teach this; that, by means of the histories of the mysteries of our Redemption, depicted by paintings or other representations, the people are instructed, and strengthened in remembering, and continually reflecting on the articles of faith; as also that great profit is derived from all sacred images, not only because the people are thereby admonished of the benefits and gifts which have been bestowed upon them by Christ, but also because the miracles of God through the means of the Saints, and their salutary examples, are set before the eyes of the faithful; that so, for those things they may give God thanks; may order their own life and manners in imitation of the Saints; and may be excited to adore and love God, and to cultivate piety. But if any one shall teach or think contrary to these decrees, let him be anathema. And if any abuses have crept in amongst these holy and salutary observances, the holy synod earnestly desires that they be utterly abolished; in such wise that no images conducive to false doctrine, and furnishing occasion of dangerous error to the uneducated, be set up. And if at times, when it shall be expedient for the unlearned people, it happen that the histories and narratives of Holy Scripture are pourtrayed and represented; the people shall be taught, that not thereby is the Divinity represented, as though it could be perceived by the eyes of the body, or be depictured by colours or figures. Moreover, in the invocation of saints, the veneration of relics, and the sacred use of images, every superstition shall be removed, all filthy lucre be abolished, finally, all lasciviousness be avoided; in such wise that figures shall not be painted or adorned with a wantonness of beauty: nor shall men also pervert the celebration of the saints, and the visitation of relics, into revellings and drunkenness; as if festivals are celebrated to the honour of the saints by luxury and wantonness. Finally, let so great care and diligence be used by bishops touching these matters, as that there appear nothing disorderly, or unbecomingly or confusedly arranged, nothing profane, nothing indecorous; since holiness becometh the house of God.
And that these things may be the more faithfully observed, the holy synod ordains, that it be lawful for no one to place, or cause to be placed, any unusual image in any place, or church, howsoever exempted, except it shall have been approved of by the bishop: also, that no new miracles are to be admitted, or new relics received, unless the said bishop has taken cognizance and approved thereof; who, as soon as he has obtained some certain information in regard of these matters shall, after having taken advice with theologians, and other pious men, act therein as he shall judge to be agreeable to truth and piety. But if any doubtful, or difficult abuse is to be extirpated, or, in fine, if any more serious question shall arise touching these matters, the bishop, before he decides the controversy, shall await the sentence of the metropolitan and of the bishops of the same province, in a provincial council; yet so, that nothing new, or that has not previously been usual in the Church, shall be decreed, without the most holy Roman Pontiff having been first consulted.
(Catechism of the Council of Trent.  Pt. IV., Chap. VI. [Buckley's Trans.])
God and the Saints addressed differently.
From God and from the Saints we implore assistance not after the same manner: for we implore God to grant us the blessing which we want, or to deliver us from evils; but the Saints, because favourites with God, we solicit to undertake our advocacy with God, to obtain of him for us those things of which we stand in need. Hence we employ two different forms of prayer: for to God, we properly say, Have mercy on us, hear us; to the saints, Pray for us.
In what Manner we may beseech the Saints to have mercy on us.
We may, however, also ask the saints themselves to have mercy on us, for they are most merciful; but we do so on a different principle, for we may beseech them that, touched with the misery of our condition, they would interpose, in our behalf, their favour and intercession with God. In the performance of this duty, it is most strictly incumbent on all, to beware lest they transfer to any creature the right which belongs exclusively to the Deity; and when we repeat before the image of any Saint the Lord's Prayer, our idea must then be to beg of the Saint to pray with us, and ask for us those favours that are contained in the form of the Lord's Prayer, to become, in fine, our interpreter and intercessor with God; for that this is an office which the saints discharge, St. John the apostle has taught in the Revelation.
The doctrine of the Eastern Church may be seen from the following from The Orthodox Confession of the faith of the Catholic and Apostolic Church of the East.
(Confes. Orthodox. P. III. Q. LII. [apud Kimmel, Libri Symbolici Ecclesiæ Orientalis  ].)
Rightly therefore do we honour the Saints of God, as it is written (Ps. cxxxix.17) "How dear are thy friends unto me, O God." And divine assistance we ask for through them, just as God ordered the friends of Job to go to his faithful servant, and that he should offer sacrifice and pray for them that they might obtain remission of sin through their patronage. And in the second place this [First] commandment forbids men to adore any creature with the veneration of adoration (latreias). For we do not honour the Saints as though adoring them, but we call upon them as our brothers, and as friends of God, and therefore we seek the divine assistance through these, our brethren. For they go between the Lord and us for our advantage. And this in no respect is opposed to this commandment of the decalogue.
Wherefore just as the Israelites did not sin when they called upon Moses to mediate between them and God, so neither do we sin, when we call for the aid and intercession of the Saints.
(Ibid. Quæstio LIV.)
This [Second] Commandment is separate from the first. For that treated of the Unity of the true God, forbidding and taking away the multitude of gods. But the present treats of external religious ceremonies. For besides the not honouring of false gods, we ought to dedicate no carved likeness in their honour, nor to venerate with adoration such things, nor to offer the sacrifices of adoration to them. Therefore they sin against this commandment who venerate idols as gods, and offer sacrifices to them, and place their whole confidence and hope in them; as also the Psalmist says (Ps. cxxxv.15), "The images of the heathen are silver and gold, etc." They also transgress this precept who are given up to covetousness, etc.
(Ibid. Quæstio LV.)
There is a great distinction between idols and images (ton eidolon kai ton eikonon). For idols are the figments and inventions of men, as the Apostle testifies when he says (1 Cor. viii.4), "We know that an idol is nothing in the world." But an image is a representation of a true thing having a real existence in the world. Thus, for example, the image of our Saviour Jesus Christ and of the holy Virgin Mary, and of all the Saints. Moreover, the Pagans venerated their idols as gods, and offered to them sacrifices, esteeming the gold and silver to be God, as did Nebuchadnezzar.
But when we honour and venerate the images, we in no way venerate the colours or the wood of which they are made; but we glorify with the veneration of dulia (douleias), those holy beings of which these are the images, making them by this means present to our minds as if we could see them with our eyes. For this reason we venerate the image of the crucifixion, and place before our minds Christ hung upon the cross for our salvation, and to such like we bow the head, and bend the knee with thanksgiving. Likewise we venerate the image of the Virgin Mary, we lift up our mind to her the most holy Mother of God, bowing both head and knees before her; calling her blessed above all men and women, with the Archangel Gabriel. The veneration, moreover, of the holy images as received in the orthodox Church, in no respect transgresses this commandment.
But this is not one and the same with that we offer to God; nor do the orthodox give it to the art of the painting, but to those very Saints whom the images represent. The Cherubim which overshadowed the mercy-seat, representing the true Cherubim which stand before God in heaven, the Israelites revered and honoured without any violation of the commandment of God, and likewise the children of Israel revered the tabernacle of witness with a suitable honour (II Sam. vi.13), and yet in no respect sinned nor set at naught this precept, but rather the more glorified God. From these considerations it is evident that when we honour the holy images, we do not transgress the commandment of the decalogue, but we most especially praise God, who is "to be admired in his Saints" (Ps. lxviii.35). But this only we should be careful of, that every image has a label, telling of what Saint it is, that thus the intention of him who venerates it may be the more easily fulfilled.
And for the greater establishment of the veneration of the holy images, the Church of God at the Seventh Ecumenical Synod anathematized all those who made war against the images, and set forth the veneration of the august images, and established it forever, as is evident from the ninth canon of that synod.
(Ibid. Quæstio LVI.)
Why was he praised in the Old Testament who broke down the brazen serpent (II Kgs. xviii.4) which long before Moses had set up on high? Answer: Because the Jews were beginning an apostasy from the veneration of the true God, venerating that serpent as the true God; and offering to it incense as the Scripture saith. Therefore wishing to cut off this evil, lest it might spread further, he broke up that serpent in order that the Israelites might have no longer that incentive to idolatry. But before they honoured the serpent with the veneration of adoration, no one was condemned in that respect nor was the serpent broken.
But Christians in no respect honour images as gods, neither in their veneration do they take anything from the true adoration due to God. Nay, rather they are led by the hand, as it were, by the image to God, while under their visible representations they honour the Saints with the veneration of dulia (doulikos) as the friends of God; asking for their mediation (mesiteuousin) to the Lord. And if perchance some have strayed, from their lack of knowledge, in their veneration, it were better to teach such an one, rather than that the veneration of the august images should be banished from the Church.
 The reader will remember that while of great weight the Catechism was not set forth by the Council, nor are its statements de fide in the Latin Church.  This is not found in Schaff's, The Creeds of Christendom, Vol. II., although part of the Orthodox Concession (viz. Pt. I.) is reprinted. The editor explains (p. 275) that he has printed "the doctrinal part in full," and has omitted the rest because it "belongs to Ethics rather than Symbolics." A somewhat extraordinary opinion to be held by anyone who has read the omitted parts.
 This is not found in Schaff's, The Creeds of Christendom, Vol. II., although part of the Orthodox Concession (viz. Pt. I.) is reprinted. The editor explains (p. 275) that he has printed "the doctrinal part in full," and has omitted the rest because it "belongs to Ethics rather than Symbolics." A somewhat extraordinary opinion to be held by anyone who has read the omitted parts.