According to Catholic doctrine, baptism confers forgiveness from all sins (original sin and personal sin) and makes the baptized Christian a temple of the Holy Spirit by giving the person sanctifying grace (CCC 1265, 1266, 1277, 1279).
One can wonder about how this teaching can allow for the experience of sin after baptism. If baptism purifies and makes us a temple of the Holy Spirit, and we receive sanctifying grace, one can wonder how they can both receive sanctifying grace and at the same time continue to sin (for Christians baptized as adults) or begin to sin (for Christians baptized as infants).
Saint Augustine provides a simple answer in his work On Baptism, namely that baptism dispels the darkness of sin, but the baptized christian can return to darkness after being baptized. Baptized Christians can pass “through the light of baptism back to their own darkness, their sins, which in the moment of their baptism had been dispelled by the holiness of the sacrament, but to which they can return.” (On Baptism I.12.19)
It is then as if the darkness that was dispelled at baptism could return, or rather the Christian could return to it. The guilt of original sin and of personal sins before baptism do not return. However, the effects of weakness of both original sin and of personal sins remain after baptism, and one must guard against this weakness or one can return to the darkness of sin. (On Baptism I.12.19)
Robert Dodaro, OSA in explaining Augustine’s view of baptism, likens baptism to the surgical removal of the guilt of original sin, and the rest of our Christian life to the convalescence and strengthening that takes place after this surgery. If one has ever experienced surgery after a broken bone, one knows how delicate the time of rehabilitation is, and after the bone is healed how crucial the strengthening which takes place during physical therapy is.
Augustine writes that the power of the sacrament to affect us depends on our disposition since “the divine excellence abides in its sacrament, whether to the salvation of those who use it well, or to the destruction of those who use it wrong” (On Baptism III.10.15). Those who use it well are those who struggle and fight against sin; this is the whole life of the saints. Those who use it wrongly, or rather who fail to use sanctifying grace, are those who do not fight against evil tendencies and temptations, but succumb to them without putting up any resistance, and sin carries them away from goodness, and they follow along willingly. (Sermon 151,7)
Augustine gives the following example: Think of two men who have been baptized, and one is worse than the other: it does not follow that the sacrament which they have is worse in the one than in the other. If one of the men is virtuous and the other one wicked, it does not follow that the baptism is bad in the evil man and good in the virtuous one; but it is good in both. Just as the light of the sun, or even of a lamp, is certainly not less bright when displayed to bad eyes than when seen by better eyes. It is the same light in both cases, although it either cheers or hurts the eyes differently according to the difference in their abilities to receive the light. (On Baptism IV.20.28)
Augustine gives yet another example that highlights the need to stand fast in the struggle against sinful tendencies and to pray for the strength to do so:
I am going to give you an example so that you can better understand. You know that there are sober men — they are few, but there are some. You also know that there are men who drink too much – these abound. Now, suppose the sober one is baptized: as far as drunkenness is concerned, he has nothing to struggle or contend with. He probably has other sinful desires to fight against, but drinking is not one of them. But, so that you understand the other cases, let us suppose now that the one baptized is the man who gets drunk often. He was baptized: all sins of drunkenness were forgiven; yet the habit remains and opposes resistance to sober living. In fact, despite being born again, this man has something to fight against. All his past vices were forgiven: yet he must pay attention, stay alert, and fight so as not to get drunk again…. If the desire to drink arises, do not give in to it; do not quench it by yielding to it, but slay it by offering resistance. Still, as long as the desire exists, it will be your enemy. If you do not consent to it and you do not drink again, the desire will wane and weaken each day that passes. If you do not have the strength to face your enemy, pray to God that you may fulfill what the Apostle commands: “Do not give in to the desires of the flesh” (Rom 13:14). What I have said about drunkenness, it is valid for all other vices and sinful tendencies. Some tendencies are inborn, and others we have acquired through a bad habit…Therefore, we ought to be always defensive against sinful tendencies, since they might decrease in us, but they never totally disappear.Saint Augustine (Sermon 151, 4-5)
The grace of God through Our Lord Jesus Christ allows us to delight in the promise of eternal life, when we will be free from the struggle against sin. While we sojourn to our heavenly homeland, grace strengthens us to engage in the necessary fight against sinful desires, and to repent and get up again if we have fallen due to our weakness, whether through an act of faith in the mercy of our Lord, if the sin is venial, or through the sacrament of reconciliation if the sin is grave.
 Lecture to Novices in November 2022
“Some tendencies are inborn, and others we have acquired through a bad habit…Therefore, we ought to be always defensive against sinful tendencies, since they might decrease in us, but they never totally disappear.“