That's an interesting question, but I must first state that I have never heart Trump speak about his faith. I have heard him reference God at various times, but not about his personal beliefs.
With that said, if you were to pose this question to someone on the left, the answer would most assuredly be that Trump has hurt Christianity's image since so many of his followers are evangelical/conservative Christians, but Trump's personal life is very un-Christian. For Christians to defend him would mean that they are ignoring or excusing bad behavior because they agree with his politics, or so the argument would go. That doesn't bode well for Christianity's image.
On the other hand, Trump's policies have been by-and-large very Christian-friendly, and so Christians have been given a larger voice in the government than they have perhaps had at any point in the last half-century or so. All one has to do is look at the trajectory of government policies under Obama (going up against the Little Sisters of the Poor, for example) and you can only imagine what it would have been like under a President Hillary. With many right-leaning judges being tapped and other Christian-friendly policies under Trump, it certainly is the case that Christianity has been helped by him.