As I have said, there are a lot of reasonable grounds to attack Carson. Religion isn’t a good one for Trump.
And in attacking him, Trump needs to do so respectfully and keep it to the issues, quite simply because Carson is a likable figure which means that an ad hominem will backfire.
And religion isn’t exactly Trump’s strong suit, so attacks on that basis, as you say, just remind people that its not Trump’s strong suit.
I happen to like Trump, he is my second choice after Cruz. Attacking Carson based on religion is a loser for him. He and Carson are going after the same group of people, the people who are tired of insiders running things. He needs to persuade people who like Carson and for that reason the tone of the attack matters.
Yeah I like Trump and Cruz too in that order.