How can the one who wills, receive the water of the blessed fountain, if it is only given to a person freely? And surely the Apostle says: It is not of the one willing nor of the one running, but of God who shows mercy (Rom. 9:16). How can one who wills receive, if he receives it freely, unless the grace of God is given for both—grace which makes a person willing from being unwilling, and then once willing, it gratuitously leads him to that which he desires?
It is as if the bountiful one should say of this same grace: Having been inspired gratuitously, he began to desire eternal things, and gratuitously he trusts that he is able to attain them. For, no one except one who wills, receives the water of life; and no one is led to eternal life freely except one who, having been first preceded by grace, begins to will. On this it is said in another passage through the excellent preacher: For, it is God who works in us both to will and to do his good pleasure (Phil. 2:13).
But the same Apostle seems to be contradictory to this opinion of his, when he says in another passage: The will is present with me, but to do good I do not find (Rom. 7:18). But it should be understood by us that he says the will is present with him, recognizing that he had divinely received this very willingness, because he also says, asking: What do you have that you did not receive? (1 Cor. 4:7). Of course, nothing whatsoever!
And so it should be said: The one who thirsts, let him come, as if it were saying: The one who, with grace preceding him, begins to desire eternal delights, should take hold of them with passionate love. And the one who wills, let him receive the water of life freely, you should understand as: The one who was made willing from being unwilling, through no preceding merits of good actions, but gratuitously by the will of God, should copiously drink the water of eternal delight from the invisible fountain.
Ambrose Autpert (730-784), Expositio in Apocalypsin. On Rev. 22:17.