Augustine of Hippo — as excerpted in the Catena Aurea on John 6:41-46

“(de Prædest. Sanctorum, c. viii. et seq.) All that are taught of God come to the Son, because they have heard and learnt from the Father of the Son: wherefore He proceeds, Every man that hath heard, and hath learned of the Father, cometh to Me. But if every one that hath heard and learnt of the Father cometh, every one that hath not heard of the Father hath not learnt. For beyond the reach of the bodily senses is this school, in which the Father is heard, and men taught to come to the Son. Here we have not to do with the carnal ear, but the ear of the heart; for here is the Son Himself, the Word by which the Father teacheth, and together with Him the Holy Spirit: the operations of the three Persons being inseparable from each other. This is attributed however principally to the Father, because from Him proceeds the Son, and the Holy Spirit. Therefore the grace which the Divine bounty imparts in secret to men’s hearts, is rejected by none from hardness of heart: seeing it is given in the first instance, in order to take away hard-heartedness. Why then does He not teach all to come to Christ? Because those whom He teaches, He teaches in mercy; and those whom He teaches not, He teaches not in judgment. But if we say, that those, whom He teaches not, wish to learn, we shall be answered, Why then is it said, Wilt thou not turn again, and quicken us? (Ps. 84:6) If God does not make willing minds out of unwilling, why prayeth the Church, according to our Lord’s command, for her persecutors? For no one can say, I believed, and therefore He called me: rather the preventing mercy of God called him, that he might believe.”