TWO GREAT MYSTERIES:

“God cannot be without man: this is a great mystery” — Pope Francis June 7, 2017

“Jesus Christ’s Gospel reveals to us that God cannot be without us: He will never be a God ‘without man’;
it is He who cannot be without us, and this is a great mystery! God cannot be God without man: this is a great mystery!”

Vatican Press & Rome Reports

We have added the emphasis above to clarify the emphasis implied in the existential reciprocity (we need God and God needs us) that Francis himself is apparently — and erroneously — attempting to convey. If God stands in need of anything, in actuality, in potentiality (potency) or in possibility, existential or otherwise, He would not be God. This is Theology 101 (the most basic theology). This is absolutely contrary to the most basic Christian (and non-Christian) concept of God.

God is the “I AM WHO AM” — the “HE WHO IS” of Exodus 3:14. He is in and of Himself being itself, self-existent, and the source of all other participated being. He is in need
of nothing and no one.

Saint Paul is clear: “Neither is He served with men's hands, as though He needed anything; seeing it is He who giveth to all life, and breath, and all things.” (Acts 17.25)

And so is the Catechism of the Catholic Church: “Without the Creator, the creature vanishes.” CCC Part I.49

Saint Thomas Aquinas explained it thus:

“God is His own existence, and not merely His own essence. ... if the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential principles. Now it is impossible for a thing's existence to be caused by its essential constituent principles, for nothing can be the sufficient cause of its own existence, if its existence is caused. Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence.” (Summa Theologica Part I Question 3 Article 4)

More simply — and much more beautifully — is this expressed by the Psalmist: “Before the mountains were brought forth, or ever Thou had formed the earth and the world, from everlasting to everlasting Thou art God.” (Psalm 90.2)