We sin from two causes: either from not seeing what we ought to do, or else from not doing what we have already seen we ought to do. Of these two, the first is ignorance of the evil; the second, weakness.
We must surely fight against both; but we shall as surely be defeated unless we are divinely helped, not only to see what we ought to do, but also, as sound judgment increases, to make our love of righteousness victor over our love of those things because of which -- either by desiring to possess them or by fearing to lose them -- we fall, open-eyed, into known sin. In this latter case, we are not only sinners -- which we are even when we sin through ignorance -- but also lawbreakers: for we do not do what we should, and we do what we know already we should not.
Accordingly, we should pray for pardon if we have sinned, as we do when we say, "Forgive us our debts as we also forgive our debtors." But we should also pray that God should guide us away from sin, and this we do when we say, "Lead us not into temptation" -- and we should make our petitions to Him of whom it is said in the psalm, "The Lord is my light and my salvation"  ; that, as Light, he may take away our ignorance, as Salvation, our weakness.
82. Now, penance itself is often omitted because of weakness, even when in Church custom there is an adequate reason why it should be performed. For shame is the fear of displeasing men, when a man loves their good opinion more than he regards judgment, which would make him humble himself in penitence. Wherefore, not only for one to repent, but also in order that he may be enabled to do so, the mercy of God is prerequisite. Otherwise, the apostle would not say of some men, "In case God giveth them repentance."  And, similarly, that Peter might be enabled to weep bitterly, the Evangelist tells, "The Lord looked at him." 
83. But the man who does not believe that sins are forgiven in the Church, who despises so great a bounty of the divine gifts and ends, and persists to his last day in such an obstinacy of mind -- that man is guilty of the unpardonable sin against the Holy Spirit, in whom Christ forgiveth sins.  I have discussed this difficult question, as clearly as I could, in a little book devoted exclusively to this very point. 
 For example, Contra Faust., XXII, 78; De pecc. meritis et remissione, I, xxxix, 70; ibid., II, xxii, 26; Quaest. in Heptateuch, 4:24; De libero arbitrio, 3:18, 55; De div. quaest., 83:26; De natura et gratia, 67:81; Contra duas ep. Pelag., I:3, 7; I:13:27.  Ps. 27:1.  II:Tim. 2:25 (mixed text).  Cf. Luke 22:61.  Cf. John 20:22, 23.  This libellus is included in Augustine's Sermons (LXXI, PL, 38, col. 445-467), to which Possidius gave the title De blasphemia in Spiritum Sanctum. English translation in N-PNF, 1st Series, Vol. VI, Sermon XXI, pp. 318-332.
 Ps. 27:1.
 II:Tim. 2:25 (mixed text).
 Cf. Luke 22:61.
 Cf. John 20:22, 23.
 This libellus is included in Augustine's Sermons (LXXI, PL, 38, col. 445-467), to which Possidius gave the title De blasphemia in Spiritum Sanctum. English translation in N-PNF, 1st Series, Vol. VI, Sermon XXI, pp. 318-332.