“All Christians believe (by definition) that Jesus of Nazareth was and is (a) God and (b) the Messiah promised by the Prophets.”
“These are factual assertions; they are objectively either true or false.”
How so? Isn’t it possible that they are subjectively either true or false?
No. "A" and "NOT-A" cannot both be true; they are contradictions. To assert one is to deny the other.
John 18:20 "I spoke openly to the world, I always taught in synagogues and in the temple, where the Jews always meet, and in secret I have said nothing."
Matthew 16:20 "Then he warned his disciples not to tell anyone that he was the Christ."
He clearly taught many things in secret and then lied to the High Priest about it.