On the words of the Gospel, John xvi.7, "I tell you the truth; it is expedient for you that I go away," etc.
1. The medicine for all the wounds of the soul, and the one propitiation for the offences of men, is to believe on Christ; nor can any one be cleansed at all, whether from original sin which he derived from Adam,  in whom all men have sinned, and become by nature children of wrath; or from the sins which they have themselves added, by not resisting the concupiscence of the flesh, but by following and serving it in unclean and injurious deeds: unless by faith they are united and compacted into His Body, who was conceived without any enticement of the flesh and deadly pleasure, and whom His Mother nourished in her womb without sin, and "Who did no sin, neither was deceit found in His Mouth."  They verily who believe on Him, become the children of God; because they are born of God by the grace of adoption, which is by the faith of Jesus Christ our Lord. Wherefore, dearly Beloved, it is with good reason that the same Lord and our Saviour mentions this one sin only, of which the Holy Ghost convinces the world, that it believeth not on Him. "I tell you the truth," He saith, "It is expedient for you that I go away. For if I go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you. And when He shall come, He will convince the world of sin, and of righteousness, and of judgment. Of sin, because they believe not on Me. Of righteousness, because I go to the Father, and ye shall see Me no more. Of judgment, because the prince of this world is already judged." 
2. Of this one only sin then He would have the world to be convinced, that they believe not on Him; to wit, because by believing on Him all sins are loosed, He would have this one imputed by which the rest are bound. And because by believing they are born of God, and become children of God; "For," saith he, "to them gave He power to become the sons of God, to them that believe on Him."  Whoso then believeth on the Son of God, in so far as he adhereth to Him, and becometh himself also by adoption a son and heir of God, and a joint-heir with Christ, in so far he sinneth not. Whence John saith, "Whosoever is born of God sinneth not."  And therefore the sin of which the world is convinced is this, that they believe not on Him. This is the sin of which He also saith, "If I had not come, they had not had sin."  For what! had they not innumerable other sins? But by His coming this one sin was added to them that believed not, by which the rest should be retained. Whereas in them that believe, because this one was wanting, it was brought to pass that all should be remitted to them that believe. Nor is it with any other view that the Apostle Paul saith, "All have sinned, and have need of the glory of God;  that "whosoever believeth on Him, should not be confounded;"  as the Psalm also saith "Come ye unto Him, and be enlightened, and your faces shall not be confounded."  Whoso then glorieth in himself shall be confounded; for he shall not be found without sins. Accordingly he only shall not be confounded who glorieth in the Lord. "For all have sinned, and have need of the glory of God." And so when he was speaking of the infidelity of the Jews, he did not say, "For if some of them have sinned, shall their sin make the faith of God of none effect?" For how should he say, "If some of them have sinned;" when he said himself, "For all have sinned"? But he said, "If some of them believed not, shall their unbelief make the faith of God of none effect?"  That he might point out more expressly this sin, by which alone the door is closed against the rest that they by the grace of God should not be remitted. Of which one sin by the coming of the Holy Ghost, that is by the gift of His grace, which is granted to the faithful, the world is convinced, in the Lord's words, "Of sin, because they believed not on Me."
3. Now there would be no great merit and glorious blessedness in believing, if the Lord had always appeared in His Risen Body to the eyes of men. The Holy Ghost then hath brought this great gift to them that should believe, that Him whom they should not see with the eyes of flesh, they might with a mind sobered from carnal desires, and inebriated with spiritual longings, sigh after. Whence it was that when that disciple who had said that he would not believe, unless he touched with the hands His Scars, after he had handled the Lord's Body, cried out as though awaking from sleep, "My Lord and my God;" the Lord said to him, "Because thou hast seen Me, thou hast believed; blessed are they that have not seen, and yet have believed."  This blessedness hath the Holy Ghost, the Comforter, brought to us, that the form of a servant which He took from the Virgin's womb, being removed from the eyes of flesh, the purified eye of the mind might be directed to This Form of God, in which He continued equal with the Father, even when He vouchsafed to appear in the Flesh; so as that with the Same Spirit filled the Apostle might say, "Though we have known Christ after the flesh; yet now we know Him so no longer."  Because even the Flesh of Christ he knew not after the flesh, but after the Spirit, who, not by touching in curiosity, but in believing assured, acknowledgeth the power of His Resurrection; not saying in his heart, "Who hath ascended into heaven? that is, to bring Christ down; or, Who hath descended into the deep? that is, to bring back Christ from the dead." "But," saith he, "the word is nigh thee, in thy mouth, that Jesus is the Lord; and if thou shalt believe in thine heart that God hath raised Him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness, and with the mouth confession is made unto salvation."  These, Brethren, are the words of the Apostle, pouring them forth with the holy inebriation of the Holy Ghost Himself.
4. Forasmuch then as we could in no way have had this blessedness by which we see not and yet believe, unless we received it of the Holy Ghost; it is with good reason said, "It is expedient for you that I go away. For if I go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you."  By His Divinity indeed He is with us always; but unless He had in Body gone away from us, we had always seen His Body after the flesh. and never believed after a spiritual sort; by the which belief justified and blessed we might attain  with cleansed hearts to contemplate the Very Word, God with God, "by whom all things were made," and "who was made Flesh, that He might dwell among us." And if not with the contact of the hand, but "with the heart man believeth unto righteousness;" with good reason is the world, which will not believe save what it sees, convinced of our righteousness. Now that we might have that righteousness of faith of which the unbelieving world should be convinced, therefore said the Lord, "Of righteousness, because I go to the Father, and ye shall see Me no more." As if He had said, "This shall be your righteousness, that ye believe on Me, the Mediator, of whom ye shall be most fully assured that He is risen again and gone to the Father, though ye see Him not after the Flesh; that by Him reconciled, ye may be able to see God after the Spirit." Whence He saith to the woman who represents the Church, when she fell at His Feet after His Resurrection, "Touch Me not, for I am not yet ascended to the Father."  Which expression is understood mystically, thus. "Believe not in Me after a carnal manner by means of bodily contact; but thou shalt believe after a spiritual manner; that is, with a spiritual faith shalt touch Me, when I shall have ascended to the Father." For, "blessed are they who do not see, and believe." And this is the righteousness of faith, of which the world, which hath it not, is convinced of us who are not without it; for "the just liveth by faith."  Whether it be then that as rising again in Him, and in Him coming to the Father, we are invisibly and in justification perfected; or that as not seeing and yet believing we live by faith, for that "the just liveth by faith;" with these meanings said He, "Of righteousness, because I go to the Father, and ye shall see Me no more."
5. Nor let the world excuse itself by this, that it is hindered by the devil from believing on Christ. For to believers the prince of the world is cast out,  that he work no more in the hearts of men whom Christ hath begun to possess by faith; as he worketh in the children of unbelief;  whom he is constantly stirring up to tempt and disturb the righteous. For because he is cast out, who once had dominion interiorly he wageth war exteriorly. Although then by means of his persecutions, "the Lord doth direct the meek in judgment;"  nevertheless in this very fact of his being cast out, is he "judged already." And of this, "judgment" is the world convinced; for in vain doth he who will not believe on Christ complain of the devil whom, judged, that is, cast out, and for the exercising of us allowed to attack us from without, not only men, but even women, and boys, and girls, Martyrs have overcome. Now in whom have they overcome, but in Him on whom they have believed, and whom seeing not, they loved, and by whose dominion in their hearts they have got rid of a most oppressive  lord. And all this by grace, by the gift, that is, of the Holy Ghost. Rightly then doth the Same Spirit convince the world, both of "sin," because it believeth not on Christ; "and of righteousness," because they who have had the will have believed, though Him on whom they believed they saw not; and by His Resurrection have hoped that themselves also should be in the resurrection perfected; "and of judgment," because if they had had the will to believe, they could be hindered by none, "for that the prince of this world hath been judged already."
 Psalm 51:5.  1 Pet. ii. 22.  John 16:7-11.  John 1:12.  1 John 3:9.  John 15:22.  Romans 3:23.  Romans 9:33.  Psalm 33:6, Sept. (xxxiv. 5, English version).  Romans 3:3.  John 20:29.  2 Corinthians 5:16.  Romans 10:6, etc.  John 16:7.  Mereremur.  John 20:17.  John 12:31.  Ephesians 2:2.  Psalm 25:9.  Pessimo.
 1 Pet. ii. 22.
 John 16:7-11.
 John 1:12.
 1 John 3:9.
 John 15:22.
 Romans 3:23.
 Romans 9:33.
 Psalm 33:6, Sept. (xxxiv. 5, English version).
 Romans 3:3.
 John 20:29.
 2 Corinthians 5:16.
 Romans 10:6, etc.
 John 16:7.
 John 20:17.
 John 12:31.
 Ephesians 2:2.
 Psalm 25:9.