I just finished The Hammer and the Cross by Harry Harrison.

I guess it's an alternate history, rather than a straight historical novel, since the premise is based around the Vikings resisting the spread of Christianity through Europe.  The main character, Shef, is English but falls in with Vikings and helps them against the English, and the church's forces that work with them.  The medieval Christian church is portrayed, probably fairly accurately, very harshly - mostly money grubbing, cruel and aloof.

It's an interesting book, though the inventiveness of the main character seems a little ahistorical at times.