That God gives Himself to man is a fundamental point of religion. Christian theology has also insisted that God gives Himself undividedly, and not merely to the church as a whole, but particularly to each individual believer. Here are three witnesses asserting that each believer can say, “God is mine” meaning, “God is wholly mine”.
Augustine, Confessions, briefly asserts the point.
O Thou Good omnipotent, who so carest for everyone of us, as if Thou caredst for him only; and so for all, as if they were but one!
Geerhardus Vos, Grace & Glory, demonstrates the point from the prophet Hosea.
In the third place the fruition of Himself granted by God to us is individual. There can be no division to it; each must of necessity receive the whole, if he is to receive it at all. This follows from the nature of the gift itself. If the gift consisted of impersonal values, either material or spiritual, the supply might be quantitatively distributed over many persons. But being, as it is, the personal favor of God, it must be poured as a whole into the receptacle of the human heart. The parable of marriage not only teaches that the covenant relation is a monogamic one, but implies besides that it is a bond binding unitary soul to soul.
And to make the matter firm, Henry P. Liddon (quoted in Spurgeon’s Treasury of David on Psalm 63), gives textual proof and theological proof.
[Of the phrase ‘O God, thou art my God’]
The word represents not a human impression, or desire, or conceit, but an aspect, a truth, a necessity of the divine nature. Man can, indeed, give himself by halves; he can bestow a little of his thought, of his heart, of his endeavour upon his brother man. In other words, man can be imperfect in his acts as he is imperfect and finite in his nature. But when God, the Perfect Being, loves the creature of his hand, he cannot thus divide his love. He must perforce love with the whole directness and strength and intensity of his Being; for He is God, and therefore incapable of partial and imperfect action. He must give Himself to the single soul with as absolute a completeness as if there were no other being besides it, and, on his side, man knows that this gift of himself by God is thus entire; and in no narrow spirit of ambitious egotism, but as grasping and representing the literal fact, he cries “My God.” (…) Therefore we find St. Paul writing to the Galatians as if his own single soul had been redeemed by the sacrifice of Calvary: “He loved me, and gave himself for me.”