What unholy ceremony does Goodman Brown encounter in the forest, and how does he respond to the offer he's given?

Goodman Brown encounters a group of people attending a baptism....

A basin was hollowed, naturally, in the rock. Did it contain water, reddened by the lurid light? or was it blood? or, perchance, a liquid flame? Herein did the shape of evil dip his hand and prepare to lay the mark of baptism upon their foreheads, that they might be partakers of the mystery of sin, more conscious of the secret guilt of others, both in deed and thought, than they could now be of their own.

I'm not sure what offer you're referring to...