God dwells and is present substantially in every soul, even in that of the greatest sinner in the world. And this kind of union is ever wrought between God and all the creatures, for in it He is preserving their being: if union of this kind were to fail them, they would at once become annihilated and would cease to be. And so, when we speak of union of the soul with God, we speak not of this substantial union which is continually being wrought, but of the union and transformation of the soul with God, which is not being wrought continually, but only when there is produced that likeness that comes from love; we shall therefore term this the union of likeness, even as that other union is called substantial or essential. The former is natural, the latter supernatural. And the latter comes to pass when the two wills -- namely that of the soul and that of God -- are conformed together in one, and there is naught in the one that repugnant to the other. And thus, when the soul rids itself totally of that which is repugnant to the Divine will and conforms not with it, it is transformed in God through love.
(Saint John of the Cross)