Origen

God moreover is He who says, I am that I am; (Ex. 3:14.) so that it is impossible that He should be called the God of those who are not. And see that He said not, I am the God of Abraham, Isaac, and Jacob, but The God of Abraham, the God of Isaac, and the God of Jacob. But in another place He said thus, The God of the Hebrews hath sent me unto thee. (Exod. 7:16.) For they who in comparison of other men are most perfect before God, have God entirely in them, wherefore He is not said to be their God in common, but of each in particular. As when we say, That farm is theirs, we shew that each of them does not own the whole of it; but when we say, That farm is his, we mean that he is owner of the whole of it. When then it is said, The God of the Hebrews, this shews their imperfection, that each of them has some small portion in God. But it is said, The God of Abraham, the God of Isaac, and the God of Jacob, because each one of these possessed God entirely. And it is to the no small honour of the Patriarehs that they lived to God.