Separation of church and state. Repeat often.
Which would be fine, if he were simply honest about his faith.
If he's a secular agnostic and doesn't know or care if there's a God, and he comes out and says so—so be it. This wouldn't be an issue. Gary Johnson, the Libertarian candidate, has said as much (he's emphasizing more of his Lutheran upbringing now but he's being very careful on how he words it).
The issue here is that Trump is blatantly putting on a weak façade of Christianity solely because he thinks it would get him votes, even though his entire lifestyle is antithetical (sorry for all the ten-dollar words, sometimes I get on a roll) to how a Christian is supposed to live: one is supposed to live cleanly, peacefully and modestly, living life as God commands to the best of our abilities, not seeking harmful Earthly pleasures, and fervently proclaiming His good news. Does Trump even come close to that? Yet he claims to be a Christian while proudly promoting lust, greed and arrogance.
It is the on-its-face hypocrisy that makes this such a sin, not so much his religion (whatever it may be) itself.
