How "far down" the European population was Christianized is the subject of a perpetual debate, one which we lack the sources to definitively answer. However, the papal bull of 1215, Omnes utriusque sexus (Everyone of both sexes) was the first mandate that all Christians confess and commune at least once a year at Easter. Most historians have concluded that the promulgation of this bull necessarily implies that the majority of Christians did not tend to confess and commune at that frequency. Whether or not they might have still attended church, and the depth of their instruction in basic Christian doctrine is unclear, but it's unlikely to have been good.

I would suggest Kevin Madigan's new Medieval Christianity as a really solid guide to the subject.