To be correct (by the Merriam Webster dictionary) Definition of church is defined as:
1 a building for public and especially Christian worship
2 the clergy or officialdom of a religious body
the word church … is put for the persons that are ordained for the ministry of the Gospel, that is to say, the clergy — J. Ayliffe
3 often capitalized :  a body or organization of religious believers: such as
a :  the whole body of Christians
So maybe this group of atheists, who want to be separate from believers, should find another word? Like "group" or "gathering?"