First of all, perhaps my terminology is dodgy here, but by nobleman I mean A member of a noble family, of noble birth and by lord I mean a ruler -- that is, the Duke of Somewhere would be a lord; his second son (no land, no title) would be a nobleman. Probably not right, but for the purpose of this question I'll stick with that.
Also, by Middle-ages I (perhaps incorrectly) refer to Europe, roughly 11th-16th Centuries.
Could a nobleman join the clergy (and become a priest or whatever -- my knowledge on church titles isn't massive)? If so, would he still be a nobleman? I mean, he'd be of noble birth (in most cases) but would he be considered a nobleman still after joining the church?
Perhaps further, could a lord join the clergy? And if so would he have to relinquish his title and lands?
I hear of, for instance, the second son of a king joining the clergy while his older brother becomes king: it's one of those things that one hears, but I cannot actually find any evidence of it -- and again, even if it's true, would he still be considered a nobleman?