And I said to him, “I should like to continue my questions.” “Speak on,” said he. And I said, “I heard, sir, some teachers maintain that there is no other repentance than that which takes place, when we descended into the water and received remission of our former sins.” He said to me, “That was sound doctrine which you heard; for that is really the case. For he who has received remission of his sins ought not to sin any more, but to live in purity. Since, however, you inquire diligently into all things, I will point this also out to you, not as giving occasion for error to those who are to believe, or have lately believed, in the Lord.
For those who have now believed, and those who are to believe, have not repentance for their sins; but they have remission of their previous sins. For to those who have been called before these days, the Lord has set repentance. For the Lord, knowing the heart, and foreknowing all things, knew the weakness of men and the manifold wiles of the devil, that he would inflict some evil on the servants of God, and would act wickedly towards them. The Lord, therefore, being merciful, has had mercy on the work of His hand, and has set repentance for them; and He has entrusted to me power over this repentance.
And therefore I say to you, that if any one is tempted by the devil, and sins after that great and holy calling in which the Lord has called His people to everlasting life, he has opportunity to repent but once. But if he should sin frequently after this, and then repent, to such a man his repentance will be of no avail; for with difficulty will he live.” And I said, “Sir, I feel that life has come back to me in listening attentively to these commandments; for I know that I shall be saved, if in future I sin no more.” And he said, “You will be saved, you and all who keep these commandments.”
Source: The Pastor (or "The Shepherd") (New Advent)