2 What then did the Lord answer to such murmurers? “Murmur not among yourselves.” As if He said, I know why you are not hungry, and do not understand nor seek after this bread. “Murmur not among yourselves: no man can come unto me, except the Father that sent me draw him.” Noble excellence of grace! No man comes unless drawn. There is whom He draws, and there is whom He draws not; why He draws one and draws not another, do not desire to judge, if you desire not to err.
Accept it at once and then understand; you are not yet drawn? Pray that you may be drawn. What do we say here, brethren? If we are “drawn” to Christ, it follows that we believe against our will; so then is force applied, not the will moved. A man can come to Church unwillingly, can approach the altar unwillingly, partake of the sacrament unwillingly: but he cannot believe unless he is willing. If we believed with the body, men might be made to believe against their will. But believing is not a thing done with the body.
Hear the apostle: “With the heart man believes unto righteousness.” And what follows? “And with the mouth confession is made unto salvation.” That confession springs from the root of the heart. Sometimes you hear a man confessing, and know not whether he believes. But you ought not to call him one confessing, if you should judge him to be one not believing. For to confess is this, to utter the thing that you have in your heart: if you have one thing in your heart, and another thing on your tongue, you are speaking, not confessing.
Since, then, with the heart man believes in Christ, which no man assuredly does against his will, and since he that is drawn seems to be as if forced against his will, how are we to solve this question, “No man comes unto me, except the Father that sent me draw him”?
Source: Tractates on the Gospel of John (New Advent)