Outside of Douglass' autobiography, Garrison wrote fairly frequently about how incompatible religion and slavery were in his newspaper, The Liberator.  He felt slavery was sin, plain and simple, and those who defended it, practiced it and encouraged it were winners, condemned to hell.

Garrison and Douglass met a number of times, and he helped Douglass to start his own newspaper, The North Star, where the former slave challenged how hypocritical slaveowners who called themselves Christians were.  It's not clear how much each of these men influenced each other's beliefs, but it is clear that Garrison was somewhat of a mentor to Douglass, and he was quite a religious man.

As far as I can tell, the name of William Lloyd Garrison only comes up once in the Narrative of the Life of Frederick Douglass.  Garrison writes the preface to the book and gives some of his beliefs there.

As the most famous abolitionist of the day, Garrison is of course strongly opposed to slavery.  He ends his preface with the slogan "No Union With Slaveholders."  That should tell you what his opinion is pretty clearly.

As for religion, all he really says about that is that slaveholders are not really Christian.  He says that Douglass shows that the ones who claim to be religious are no better than the others.  He says that anyone on the side of slavery is an enemy of God.