Augustine of Hippo
I say this concerning those predestined for the Kingdom of God, whose number is so fixed that no one is added to them and no one is taken away. I do not mean those who are many. For it can be said that they are called but not chosen, since they are not called according to His purpose. Indeed, the number of the elect is fixed, neither increasing nor decreasing. John the Baptist also points to this when he says: "Bear fruit worthy of repentance, and do not think to say within yourselves, 'We have Abraham as our father.' For God is able from these stones to raise up children to Abraham" (Mt 3:8-9). As he shows, they must be cut off if they do not bear fruit, so that the number of those promised to Abraham may be fulfilled. Yet it is stated more clearly in Revelation: "Hold fast what you have, lest someone take your crown." For if another takes it only when the first loses it, then the number is fixed.