7 Take good heed, then, to these fearful words of the great apostle; and when you feel that you do not understand, put your faith in the meanwhile in the inspired word of God, and believe both that man's will is free, and that there is also God's grace, without whose help man's free will can neither be turned towards God, nor make any progress in God. And what you piously believe, that pray that you may have a wise understanding of. And, indeed, it is for this very purpose,--that is, that we may have a wise understanding, that there is a free will.
For unless we understood and were wise with a free will, it would not be enjoined to us in the words of Scripture, "Understand now, you simple among the people; and you fools, at length be wise," The very precept and injunction which calls on us to be intelligent and wise, requires also our obedience; and we could exercise no obedience without free will. But if it were in our power to obey this precept to be understanding and wise by free will, without the help of God's grace, it would be unnecessary to say to God, "Give me understanding, that I may learn Your commandments;" nor would it have been written in the gospel, "Then opened He their understanding, that they might understand the Scriptures;" nor should the Apostle James address us in such words as, "If any of you lack wisdom, let him ask of God, who gives to all men liberally, and upbraids not; and it shall be given him."
But the Lord is able to grant, both to you and to us, that we may rejoice over very speedy tidings of your peace and pious unanimity. I send you greeting, not in my own name only, but of the brethren also who are with me; and I ask you to pray for us with one accord and with all earnestness. The Lord be with you.
Source: Letters (New Advent)