Yes, I guess I am unsure if you want a word for the speech act or the speaker or the speech in general. That being said, I'll try to cover all three (the first example being specifically non-religious; the rest carrying religious weight.
A challenger in a specific context: An Agitator speaks to agitate a group or a people. Their speech act is agitation.
A challenger in a religious context: A Heretic speaks heresy against the church. They are heretical before the Church.
Other words that describe such a person without as strong a force "Heretic" conveys, for example, (the person who speaks - the idea/position - the type of speech) as used in the context of a Church:
Apostate - Apostasy - to Apostatize (to dissent, to denigrate)
Sectarian - Sectarianism - no direct verb (to imprecate, to execrate)
Pagan - Paganism - "Paganry," while a word I've heard and used for its humor....apparently is not a word (we can change that!!!), but to speak as though one were a pagan, basically any speech form that offends the church works - "to blaspheme" is probably the most applicable for someone who engages in "Paganry," as a specific speech act.