9 The Apostle Paul, rapidly recounting the benefits of God, ended with the words, “And who is sufficient for these things?” Wherefore, also, in another place he says, “Such confidence have we through Christ to Godward; not that we are sufficient of ourselves to think anything as of ourselves; but our sufficiency is from God; Who also made us sufficient as ministers of a new covenant; not of the letter but of the spirit; for the letter kills, but the spirit gives life.”
Do we still dare to pride ourselves on free will, and to abuse the benefits of God to the dishonour of the giver? Whereas the same chosen vessel openly writes, “We have this treasure in earthen vessels, that the exceeding greatness of the power may be of God, and not from ourselves.” Therefore, also, in another place, checking the impudence of the heretics, he says, “He that glories, let him glory in the Lord. For not he that commends himself is approved, but whom the Lord commends.”
And again, “In nothing was I behind the very chiefest Apostles, though I be nothing.” Peter, disturbed by the greatness of the miracles he witnessed, said to the Lord, “Depart from me, for I am a sinful man.” And the Lord said to His disciples, “I am the vine and you are the branches: He that abides in Me and I in him, the same bears much fruit, for apart from Me ye can do nothing.” Just as the vine branches and shoots immediately decay when they are severed from the parent stem, so all the strength of men fades and perishes, if it be bereft of the help of God.
“No one,” He says, “can come unto Me except the Father Who sent Me draw him.” When He says, “No one can come unto Me,” He shatters the pride of free will; because, even if a man will to go to Christ, except that be realized which follows— “unless My heavenly Father draw him”— desire is to no purpose, and effort is in vain. At the same time it is to be noted that he who is drawn does not run freely, but is led along either because he holds back and is sluggish, or because he is reluctant to go.
Source: Against the Pelagians (New Advent)