Grace and free will, which comes first?
While Catholics believe in human freedom it should be noted that we can exercise our freedom after being moved first by God’s Grace, i.e. we can choose whether to cooperate with His Grace or to reject it.
When God touches man’s heart through the illumination of the Holy Spirit, man himself is not inactive while receiving that inspiration, since he could reject it; and yet, without God’s grace, he cannot by his own free will move himself toward justice in God’s sight.
Catechism of the Catholic Church # 1993 (underlined emphasis added)
Thus Catholics believe that God, not us, who takes the first initiative in our salvation. Those who believe that we, in using our freedom, can initiate our salvation and then God will assist us by giving us grace follow the teaching of Pelagian or Semi-Pelagian. On this issue Augustine wrote (English translation from Nicene and Post-Nicene Fathers or NPNF series I, Vol. 5, available online at http://www.ccel.org):
When God says, “Turn ye unto me, and I will turn unto you,” [Zechariah 1:3] one of these clauses-that which invites our return to God-evidently belongs to our will; while the other, which promises His return to us, belongs to His grace. Here, possibly, the Pelagians think they have a justification for their opinion which they so prominently advance, that God’s grace is given according to our merits. In the East, indeed, that is to say, in the province of Palestine, in which is the city of Jerusalem, Pelagius, when examined in person by the bishop, did not venture to affirm this. For it happened that among the objections which were brought up against him, this in particular was objected, that he maintained that the grace of God was given according to our merits,-an opinion which was so diverse from catholic doctrine, and so hostile to the grace of Christ, that unless he had anathematized it, as laid to his charge, he himself must have been anathematized on its account. He pronounced, indeed, the required anathema upon the dogma, but how insincerely his later books plainly show; for in them he maintains absolutely no other opinion than that the grace of God is given according to our merits. Such passages do they collect out of the Scriptures,-like the one which I just now quoted, “Turn ye unto me, and I will turn unto you,”-as if it were owing to the merit of our turning to God that His grace were given us, wherein He Himself even turns unto us. Now the persons who hold this opinion fail to observe that, unless our turning to God were itself God’s gift, it would not be said to Him in prayer, “Turn us again, O God of hosts [Psalms 80:7];”.and, “Thou, O God, wilt turn and quicken us [Psalms 85:6];.and again, “Turn us, O God of our salvation [Psalms 85:4],”-with other passages of similar import, too numerous to mention here. For, with respect to our coming unto Christ, what else does it mean than our being turned to Him by believing? And yet He says: “No man can come unto me, except it were given unto him of my Father. [John 6:65]“.
Augustine, A Treatise on Grace and Freewill, Chapter 10