Answer:
"The incarnation of Jesus"
Explanation:
in other words, the argument was that Jesus, one of the members of the trinity, was "made of flesh and bone" and therefore could be painted, unlike the other parts of the trinity which are argued to be "of the spirit"
in other words, if I may be so brash in my wording, people were more or less like
"you can't draw what you can't see!"
and the catholic church was like
"BUT YOU CAN SEE JESUS, HE HAS A BODY"
(sorry but I just found the simplified argument a bit funny, I'm sitting here imagining my siblings arguing about if ghosts are real again...)
