I myself have witnessed the truth of this. I had the good fortune of living with many knowers of God. I and others knew them through the years, and one of the most beautiful things we noticed was that although they were knowers of God even in their youth, as time went on there came a sort of expansion of their being. In several cases the expansion was so great that almost within one year their personality seemed to have totally changed; they manifested qualities much greater than those who had earlier been considered greater than they. Tremendous expansion took place. Tremendous! Now, how do you account for that? The only way you can account for it is by admitting that if your mind dwells in that higher plane, the divine plane, and you resist the limitations contrary to divine nature, then even though you may live on the relative plane, gradually you will find that a more expansive nature, which is essentially divine, begins to assert itself. Powers come, ability comes; if such a person says something it will be fulfilled. So, you see, some Vedantists say, How can you distinguish such people from a Divine Incarnation? They are Incarnations.
Swami Vivekananda once enunciated a principle by which you can distinguish Divine Incarnations from other saints and sages. He said that they are characterized by tremendous compassion, and he designated as Incarnations of God some saints who had never before been looked upon as such and spoke of one or two who have been looked upon as Divine Incarnations as just saints or prophets or scholars. ” Now, of course, the majority won't agree with that assessment of his, but when a Swami Vivekananda makes a statement in connection with such subjects, his words have to be accepted for at least serious consideration. Anyhow, his criterion was compassion. The heart of a person becomes so large, he feels so at one with all, that you cannot distinguish him from an Incarnation. Even if originally you didn't think him to be an Incarnation, he has now become like one, and when he is like, then he is. You cannot stick to like' for long, you see. You begin to ask, Why do I call him like? The right substance is there: real compassion, endless compassion, is there; so why should I say it is like.
The Complete Works of Swami Vivekananda, 6:393-94 (9th edition). Universal compassion? It is the same universal compassion that is ascribed to a Divine Incarnation.
But as I said, Vedantists don't want to deny any fact or truth. So if you aren't an extreme monist, that is to say, if you aren't absorbed in contemplation of the attributeless Brahman Brahman beyond anything that is relative then you will find that the idea of Divine Incarnation as someone apart has a great deal of practical justification; you can easily accept it. From a theoretical standpoint, also, there is no difficulty about the concept of Divine Incarnation, if you think, as do all Vedantists of whatever school, that there is an Isvara or God, a Lord of this universe.