According to traditional Christian doctrine, the Son of God shares the Divinity of the Father, and for this reason, or to this extent, the Son may be said to be “equal” to the Father. On the other hand—as I’ve pointed out several times in this forum—the Son qua Person is not the equal of the Father qua Person, and He Himself says so: “My Father is greater than I” (John 14:28). Metaphysically speaking, there is therefore something relatively determinate—“relatively Absolute”—about the Divine Son in comparison with the Fons et Origo from whom His Divinity is derived, that is, the Father.
Basing yourself on these Christological axioms, you pose a question. Citing Metropolitan Kallistos—“to be deified is to be ‘christified’: the divine likeness that we are called to attain is the likeness of Christ” (The Orthodox Way, 74)—you want to know what “level” or “degree” of Divinity a man may expect, or rather hope, to share in as a result of theosis.
No doubt Christianity offers us the possibility of becoming all that Christ is (Ephesians 4:13) and of having exactly the same relationship as He has with the Father (John 17:21). But if there’s something “greater than” the Son, Divine though He is, must there not also be something greater than the state enjoyed by the saint, deified though he is? And if so—you ask—doesn’t it follow that a Christian is obliged to disavow the Hindu’s goal of realizing the Identity, or better Ipseity, designated by the Upanishadic maxim Tat tvam asi?
This is a clever, and no doubt inevitable, question, but it seems to me you’re allowing yourself to be mesmerized by the terminology and forgetting that the teachings of the world’s saving traditions aren’t meant to match up in some sort of one-for-one way. Dogmas are essential, of course, but we need to make sure we don’t allow them to trap us in a sort of Zeno-like paralysis, where we end up convinced we can’t cross the lines with which we ourselves have divided the Path into parts.
Perennialists contend that there is a transcendent, not an immanent, unity of religions—a Summit located in the Divine “stratosphere” and not the human “atmosphere”. Twist and turn our words as we might, I freely admit we’ll never find a way of mapping the language of Trinitarian theology onto the language of Advaita Vedānta. Nonetheless it’s obvious—is it not?—that three-in-oneness and not-two-ness are inconnumerably pointing us in the same direction. It is for us to move.