Out of the 27 books of the New Testament, Paul wrote half of them (13). His weight in the propagation of Christianity is even higher when you consider that most scholars think the gospels (the first four books of Matthew, Mark, Luke & John) are all derivatives of a different singular source.
Most of the modern tenet of (conservative) Christianity derive from Paulist writings, not from anything allegedly attributed to Jesus. Paul is the one whose decrees against homosexuality and keeping women in their place are most found in the New Testament.
And Paul was NOT even one of the followers of Jesus during his lifetime. That whole "he used to persecute Christians then was blinded on the road to Damascus and became a believer" schtick years after Jesus allegedly died is simply not believable.
Christianity is barely "christian" and is actually a "Paulist" religious sect.
I am skeptical Paul would "write a letter" - the Christians that support Trump are just the people literally following Paul's mysoginy and homophobia.
