In the course of his commentary on John 6, Aquinas writes,
938 A general reason can be given why God does not draw all who are turned away from him, but certain ones, even though all are equally turned away. The reason is so that the order of divine justice may appear and shine forth in those who are not drawn, while the immensity of the divine mercy may appear and shine in those who are drawn. But as to why in particular he draws this person and does not draw that person, there is no reason except the pleasure of the divine will. So Augustine says: “Whom he draws and whom he does not draw, why he draws one and does not draw another, do not desire to judge if you do not wish to err. But accept and understand: If you are not yet drawn, then pray that you may be drawn.” We can illustrate this by an example. One can give as the reason why a builder puts some stones at the bottom, and others at the top and sides, that it is the arrangement of the house, whose completion requires this. But why he puts these particular stones here, and those over there, this depends on his mere will. Thus it is that the prime reason for the arrangement is referred to the will of the builder. So God, for the completion of the universe, draws certain ones in order that his mercy may appear in them; and others he does not draw in order that his justice may be shown in them. But that he draws these and does not draw those, depends on the pleasure of his will. In the same way, the reason why in his Church he made some apostles, some confessors, and others martyrs, is for the beauty and completion of the Church. But why he made Peter an apostle, and Stephen a martyr, and Nicholas a confessor, the only reason is his will. We are now clear on the limitations of our human ability, and the assistance given to us by divine help.