He was.
Jesus Christ is, and was, 100% God. So it is fanciful in the extreme to suppose that a mere mortal, of whom God said, "Dust you are, to dust you will return," can effectively transfigure himself. If humanity is to be 'transfigured', glorified on the last day, it must first take the narrow path that leads to life, that begins with the cross of Christ.
'I resolved to know nothing while I was with you except Jesus Christ, and him crucified.' 1 Co 2:2-3
Crucified, not transfigured. Transfiguration is mentioned only once in the NT letters, and that in order to demonstrate that Jesus was
not merely human- the precise opposite of the argument presented here.
Muslims say that the crucifixion was null and void- and Eastern Orthodox agree, though less openly. Eastern Orthodoxy and Islam also agree that, by effort- fasting, prayer etc.- one can reach a state of acceptance by deity. Christianity says that this is very far from reality. It says that acceptance of the offence of the cross is essential for a new nature to be born, and good works done without that total conversion of the will are no better than whitewash.
So yes, Eastern Orthodoxy and Islam could easily merge- but they will not, because each would lose more than it would gain. Muslims would not accept even the nominality of the Christianity of the EO, because it would seem like a basic theological concession, to the average person, who understands very little, and barely wishes to understand what little is understood. EOs in Mediterranean countries would think they would taint themselves by association with 'infidels', few of them aware that they hold similar beliefs.