Mistah Chick -- he dead
Chick's work expresses a crude and simple form of Christianity. If you accept Jesus before you die, you go to Heaven; otherwise you go to Hell regardless of your sins or virtues, with no thought given to explaining why God would make our lives such a simplistic and unjust game for such high yet invisible stakes. The world is massively infested with demons -- homosexuality, drug use, pop culture, anything "occult", all non-Christian religions, and all forms of Christianity deviating from Chick's own (especially Catholicism) -- all are the work of demons. "Illicit" sex is condemned, though that subject doesn't come up as often as one might expect. Any character representing anything Chick doesn't like is mono-dimensionally evil and usually absurdly short-tempered.
There are several distinct art styles, and most likely several artists worked on the comics over the years.
The comics' subject matter varies considerably. Some are simple retellings of Bible stories, many are tales of an individual being offered the chance to accept Jesus and either taking or rejecting it (though the circumstances shown in these tales are very varied), some are stories of sinners who either reform or come to a bad end, a few are visions of an apocalyptic future where Christians are viciously persecuted by a secular world. Some of the comics can be pretty entertaining if you are in the right frame of mind. A personal favorite image of mine from Chick's work is this one:
It's hard to believe many people were actually converted by Chick tracts. The world they depict is so blatantly at odds with reality that one imagines Chick must have been some sort of hermit. Smart propagandists work to understand the people they're trying to convert. Chick seems to have known very little about anyone different from himself. The tracts are widely sold by secular vendors to secular customers for entertainment -- which is how I got my own large collection, including the ones scanned for the image at the top of this post.