Grace and human freedom
The Catechism of the Catholic Church declares (# 2001): The preparation of man for the reception of grace is already a work of grace. God through His grace always takes the initiative for our salvation. This vocation of eternal life is supernatural. It depends entirely ob God’s gratuitous initiative (ibid # 1998). We, in using our freedom, respond to it. God’s free initiative demands man’s free response (ibid # 2002). Thus in Catholic teaching we have freedom to choose between good and evil (ibid # 1732); yet the Catechism also reminds us that without God’s grace, he [man] cannot by his own free will move himself toward justice in God’s sight (ibid # 1993). We cannot do anything good without first being moved by God’s grace.
Now the Lord Himself not only shows us what evil we should shun, and what good we should do, which is all that the letter of the law is able to effect; but He moreover helps us that we may shun evil and do good [Psalms 37:27], which none can do without the Spirit of grace; and if this be wanting, the law comes in merely to make us guilty and to slay us. It is on this account that the apostle says, “The letter killeth, but the Spirit giveth life [2 Corinthians 3:6]. He, then, who lawfully uses the law learns therein evil and good, and, not trusting in his own strength, flees to grace, by the help of which he may shun evil and do good. But who is there who flees to grace except when “the steps of a man are ordered by the Lord, and He shall determine his way”? [Psalms 37:23] And thus also to desire the help of grace is the beginning of grace; of which, says he, “And I said, Now I have begun; this is the change of the right hand of the Most High.” [Psalms 76:10]. It is to be confessed, therefore, that we have free choice to do both evil and good; but in doing evil every one is free from righteousness and a servant of sin, while in doing good no one can be free, unless he have been made free by Him who said, “If the Son shall make you free, then you shall be free indeed.” [John 8:36]. Neither is it thus, that when any one has been made free from the dominion of sin, he no longer needs the help of his Deliverer; but rather thus, that hearing from Him, “Without me ye can do nothing,” [John 15:5]. he himself also says to Him, “Be thou my helper! Forsake me not.” [Psalms 27:9]. I rejoice that I have found in our brother Florus also this faith, which without doubt is the true and prophetical and apostolical and catholic faith; whence those are the rather to be corrected-whom indeed I now think to have been corrected by the favour of God-who did not understand him.
Augustine, A Treatise on Rebuke and Grace, Chapter 2 (underlined emphasis is added)
English translation from Nicene and Post Nicene Fathers (NPNF), Series I, Vol. 5
(available online at http://www.ccel.org)