Do you mean:
a) Is there any reason for not believing that Jesus Christ was the son of God? Or
b) Is there any reason for not believing that someone called Jesus actually existed all those years ago?
Scholors and historians, I understand, are united in their affirmation that somebody, some human, named Jesus did walk the earth and did exist.
Human beings though, are divided on whether such a man was the living and breathing mesiah and son of God. Some are unconvinced that he was the son of God, because they believe such thought is dellusionary, make-believe, bizarre, chinese whispers etc.
That opinion is valid. Afterall, there are all sorts of bigoted, angry, unloving, infeasable stories in the Bible.
Others, think that Jesus may have been only human, but that HIS MESSAGE was holy, loving and offered salvation for mankind.
Truly this subject is a bit of a minefield.