If he knows some medieval theology
Royal Canadian Bandit's answer mentioned him going to a monastery where they would end up valuing his skill to read and write. Now, it just occurred to me that if he's a christian and followed higher level secondary education with Latin (this is still the norm in a lot of developed countries) and at some point he read up on some medieval theology he would definitely be taken seriously by everybody. His knowledge wouldn't even have to be extremely in-depth. Like having spend a couple of afternoon reading Wikipedia articles or the excellent Internet Encyclopedia of Philosophy would probably been enough.
The interesting thing to understand is that the sciences were traditionally seen as sub-categories of theology (which sounds totally ridiculous to us nowadays, but the natural being sub categorized to the supernatural actually makes a lot of sense in such a society). So if he can establish some basic authority as a good theologian (claiming he's a wandering monk or what not, something that is technically true as he just wandered from the present to the past), he can next start thinking about the other sciences. And depending on what he still remembers work from there.
If he does not know anything about medieval theology
On the other hand, if he doesn't know the above stuff (quite likely) he won't be destined for any greatness. If he accepts being a normal peasant he will probably do just fine, if however he tries to become self important and introduce modern technologies and stuff that will soon be the end of him (nothing dramatic like being burned on the stake, just nobody willing to trade with him or help him*). And yeah, if he's a christian (or is capable to act like one), but not all that learned then just going to a monastery would be the wisest choice.
* Most of medieval times weren't half as barbaric as depicted in modern media. Realistically it depends on what he would say though. If he starts proclaiming post modernist notions or even modernist notions the Church would take interest probably and excommunicate him. But besides that he would be more likely treated like an idiot than 'burned on a stake' like some of the other answers suggested.
PS. This is assuming that language is not an issue, as that's how I understood the question. And just for the record, it's not that faith was that much more important in that time, but there are only two theoretical powers where he could flourish: political or theology. The first requires the right bloodline and the second requires knowledge.