Why might a magus be Christian? Because he can tell that the priest has no magical powers to speak of; InVi shows nothing. He knows that before the Mass, the host is bread. InHe is very clear on the matter. He can sit in the church with a Sun duration InHe spell up, watching that bread.
At the moment of consecration, the spell stops telling him anything about the Host.
This happens every single time he tries it. Sneaking a bit of the bread off before consecration to get a massively high Penetration total is no help.
Now, the priest says that the Host is God after consecration. That's certainly consistent with what the magus can detect, reliably, at least once per week.
Sure, there are other explanations available, but an ordained priest, any ordained priest, but only an ordained priest, saying those words can always turn bread into... something that is immune to magic. (ArM5 page 203)
As far as I recall from Lucretius (it's a long time since I read De Rerum Natura), one of the arguments against gods in classical atheism is that you just don't see the miracles you'd expect if they were there and involved.
In ME, however, you do see those miracles.
Atheism is a much, much less attractive position in ME than it is in the real world; it has a lot of evidence to explain away. Deism has problems with exactly the same evidence, as it seems to indicate that there is a tinkering God. Historically, hardly anyone has been atheist; theism is far more common. Thus, in ME I suspect that the majority of magi will be approximately Christian. I mean, they have the evidence on their side.
It is certainly true that they can find apparent logical contradictions within the evidence, but that probably won't lead them to doubt the overall picture. After all, scientists live with the fact that general relativity and quantum mechanics are logically incompatible, and don't generally accept the new theories that can make them compatible, due to lack of evidence. I can see magi taking the same attitude: Judaism, Christianity, and Islam all appear to be true, but they aren't consistent. Therefore, we don't understand something. However, rejecting the evidence that the three religions are all true is not a sensible response.