Time travel .. In which direction?
To the past, as shown by both the General and Special Theories of Relativity.
The funny thing about time travel, if indeed it's truly possible, is that it will end up proving predestination, so I don't see it as being any particular challenge to Christianity, since God exists outside of the created order.
If it's going to happen it already has. It's a part of history, whether traveling into the past or returning from the future. If the one is possible then so is the other; they're dependent upon one another. Time is not so fragile as science fiction portrays. A future exists that culminated in the time traveler, no matter what action that time traveler might undertake. A past that contains a time traveler is a given in order for one to exist at all.
But only if it's possible, of course. I believe it is. Barring some scientific breakthrough beforehand, we or a subsequent generation will see it with the two witnesses.