Does theosis equal salvation? I don't know. Does it equal the mercy of God, I don't know. Perhaps we talk about theosis too much, as if it were something in the abstract, and not a dynamic mystery. I believe schism and heresy can offer hindrances, but God is trying to save everyone, not just the Orthodox. And for those who do not have Orthodoxy, whether they be pagans, Jews, heterodox, or deluded Orthodox, there is one word that, my priest says, is not in God's dictionary--hopeless.
St. Silouan the Athonite, IIRC, was troubled over the salvation the Orthodox, the non-Orthodox, indeed all mankind for he was a great saint. Who can tell the vigils and prayers he made on behalf of all men? In prayer, being troubled very much in spirit about the salvation of mankind, the Lord Christ appeared to him that He would have mercy upon each person who asked Him for mercy even once. St. Silouan then asked, what about those who had left all to follow the Lord? The Lord said that these would be His friends, the others he would merely have mercy upon.
Merely. This is not any small thing. The mercy of God is deeper than the ocean, broader than the sky, higher than the heavens, sweeter than honey, and better than life itself, or even many lives. It endures forever.
So, about theosis, I don't even know about myself. But I ask God to have mercy on me and all mankind.