12 C. Granted that no one could avoid all sin in boyhood, youth, and early manhood; can you deny that very many righteous and holy men, after falling into vice, have heartily devoted themselves to the acquisition of virtue and through these have escaped sin?
A. This is what I told you at the beginning— that it rests with ourselves either to sin or not to sin, and to put the hand either to good or evil; and thus free will is preserved, but according to circumstances, time, and the state of human frailty; we maintain, however, that perpetual freedom from sin is reserved for God only, and for Him Who being the Word was made flesh without incurring the defects and the sins of the flesh. And, because I am able to avoid sin for a short time, you cannot logically infer that I am able to do so continually. Can I fast, watch, walk, sing, sit, sleep perpetually?
C. Why then in Holy Scripture are we stimulated to aim at perfect righteousness? For example: “Blessed are the pure in heart, for they shall see God,” and “Blessed are the undefiled in the way, who walk in the law of the Lord.” And God says to Abraham, “I am your God, be thou pleasing in My sight, and be thou without spot, or blame, and I will make My covenant between Me and you, and will multiply you exceedingly.” If that is impossible which Scripture testifies, it was useless to command it to be done.
A. You play upon Scripture until you wear a question threadbare, and remind me of the platform tricks of a conjurer who assumes a variety of characters, and is now Mars, next moment Venus; so that he who was at first all sternness and ferocity is dissolved into feminine softness. For the objection you now raise with an air of novelty— “Blessed are the pure in heart,” “Blessed are the undefiled in the way,” and “Be without spot,” and so forth— is refuted when the Apostle replies, “We know in part, and we prophesy in part,” and, “Now we see through a mirror darkly, but when that which is perfect has come, that which is in part shall be done away.” And therefore we have but the shadow and likeness of the pure heart, which hereafter is destined to see God, and, free from spot or stain, to live with Abraham. However great the patriarch, prophet, or Apostle may be, it is said to them, in the words of our Lord and Saviour, “If you being evil, know how to give good gifts unto your children, how much more shall your Father Which is in heaven give good things to them which ask Him?” Then again even Abraham, to whom it was said, “Be thou without spot and blame,” in the consciousness of his frailty fell upon his face to the earth. And when God had spoken to Him, saying, “Your wife Sarai shall no longer be called Sarai, but Sara shall her name be, and I will give you a son by her, and I will bless him and he shall become a great nation, and kings of nations shall spring from him,” the narrative at once proceeds to say, “Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? And shall Sarah, that is ninety years old, bear?” And Abraham said unto God, “Oh, that Ishmael might live before you!” And God said, “Nay, but Sarah your wife shall bear you a son, and you shall call his name Isaac,” and so on. He certainly had heard the words of God, “I am your God, be thou pleasing in My sight, and without spot”; why then did he not believe what God promised, and why did he laugh in his heart, thinking that he escaped the notice of God, and not daring to laugh openly? Moreover he gives the reasons for his unbelief, and says, “How is it possible for a man that is an hundred years old to beget a son of a wife that is ninety years old?” “Oh, that Ishmael might live before you,” he says. “Ishmael whom thou once gavest me. I do not ask a hard thing, I am content with the blessing I have received.” God convinced him by a mysterious reply. He said, “Yea.” The meaning is, that shall come to pass which you think shall not be. Your wife Sara shall bear you a son, and before she conceives, before he is born, I will give the boy a name. For, from your error in secretly laughing, your son shall be called Isaac, that is laughter. But if you think that God is seen by those who are pure in heart in this world, why did Moses, who had previously said, “I have seen the Lord face to face, and my life is preserved,” afterwards entreat that he might see him distinctly? And because he said that he had seen God, the Lord told him, “You can not see My face. For man shall not see My face, and live.” Wherefore also the Apostle calls Him the only invisible God, Who dwells in light unapproachable, and Whom no man has seen, nor can see. And the Evangelist John in holy accents testifies, saying, “No man has at any time seen God. The only begotten Son Who is in the bosom of the Father, He has declared Him.” He Who sees, also declares, not how great He is Who is seen, nor how much He knows Who declares; but as much as the capacity of mortals can receive.
Source: Against the Pelagians (New Advent)