What you call "the Trinity and the Catholic church's definition
of it" is precisely the result of the first Christians getting
together and trying to find an acceptable answer to your
question "what is a Christian?". I can't see what you are
objecting to: someone is saying what historians of all beliefs
would agree on, and you are calling him a closed-minded bigot?

You really ought to say what you mean by "belief in Jesus
Christ". It is not a wording that is sufficient to describe a
Christian. Muslims believe in Jesus Christ although they
believe he was a prophet and not the incarnated Son of God. But
followers of Eastern religions might be quite happy to say that
Jesus was the incarnation of God - along with large numbers of
other historical and mythical figures.

So perhaps you ought to rephrase your question and say
precisely what it is in the traditional definitions of what it
is to be a Christian, as handed down by the Universal Church,
you object to but regard as unnecessary for being a Christian.