Depends on the religion. He may suffer from a sect that's just so looked-down-upon that he dare not admit to it in public. Quaker, Shaker, a follower of Halle-Bopp or Jim Jones, atheist...
That's very unlikely, because much of his behavior has been on the spotlight for a very long time, and there was never anything that even suggested suspected any of that.
For example, he wants to have followers, not to be a follower. And he's good at cons. He would almost certainly not fall for such an obvious one.
Also, his behavior so far indicates he just lies to get whatever he wants, and that's power, money, public recognition, etc., not the kind of thing you'd get from a follower of those religions.
If you count atheism as a religion, surely he's not a zealot. He's not even been caught arguing for atheism, or doing anything to deconvert people, etc.
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He's pretty bad, but let's not exaggerate.
As for bad character traits he doesn't possess, he's not a serial killer. Or a child molester. Or a religious zealot. And so on.
He would be a terrible POTUS, though.
With the number of reports of him and his friends pressuring underage girls into sex in the 1980's and his behavior towards his own daughter even today, he's pretty damned close to a child molester.
And his running mate is the religious zealot.
So we are left with - "he's not a serial killer". That should be his slogan: "I'm not a serial killer"
His running mate is a tactical choice, to get votes from the Christian right. As for the reports, could you please cite some credible source?
As for his own daughter, she's not remotely a child, and there is no good reason at all to think he would molest her.
Besides, no, it wouldn't be just that he's not a serial killer. That was an example. Most crimes have not been committed by Trump, and he's not shown any inclinations to commit them.
It's remarkable how even when they get a really bad opponent, so many people accuse him without warrant, despite the easy availability of warranted accusations to make.