From the Roman Catechism for the 6th Sunday after Pentecost

From the Epistle of St. Paul to the Romans on the Sixth Sunday after Pentecost:

Know you not that all we, who are baptized in Christ Jesus, are baptized in his death? For we are buried together with him by baptism into death; that as Christ is risen from the dead by the glory of the Father, so we also may walk in newness of life. -Romans 6:3-4


Frequent reflection upon these truths cannot fail, in the first place, to fill the minds of the faithful with admiration for the infinite goodness of God, who, uninfluenced by any other consideration than that of His mercy, gratuitously bestowed upon us, undeserving as we are, a blessing so extraordinary and divine as that of Baptism. To inflame the minds of the faithful, however, with a zeal for true piety, pastors will find no means more efficacious than an accurate exposition of the effects of Baptism.

The effects of Baptism should be frequently explained, in order that the faithful may be rendered more sensible of the high dignity to which they have been raised, and may never suffer themselves to be cast down therefrom by the snares or assaults of Satan. 

They are to be taught, in the first place, that such is the admirable efficacy of this Sacrament that it remits original sin and actual guilt, however unthinkable its enormity may seem.This was foretold long before by Ezechiel, through whom God said: I will pour upon you clean water, and you shall be cleansed from all your filthiness. The Apostle also, writing to the Corinthians, after having enumerated a long catalogue of sins, adds: such you were, but you are washed, but you are sanctified.
That such was at all times the doctrine handed down by holy Church is clear. By the generation of the flesh, says St. Augustine in his book On the Baptism of Infants, we contract original sin only; by the regeneration of the Spirit, we obtain forgiveness not only of original, but also of actual sins. St. Jerome also, writing to Oceanus, says: all sins are forgiven in Baptism.

To remove all further doubt on the subject, the Council of Trent, after other Councils had defined this, declared it anew, pronouncing anathema against those who should presume to think otherwise, or should dare to assert that although sin is forgiven in Baptism, it is not entirely removed or totally eradicated, but is cut away in such a manner as to leave its roots still fixed in the soul. To use the words of the same holy Council, God hates nothing in those who are regenerated; for there remains nothing deserving of condemnation in those who are truly buried with Christ by Baptism unto death, "who walk not according to the flesh" but putting off the old man, and putting on the new, who is created according to God, become innocent, spotless, pure, upright, and beloved of God.