Given the extensive focus on religion, it can be logical to compare the snakes from the film to the one from the Bible. I'm of course referring to the snake that convinces Eve to eat the apple from the Tree of Knowledge. An excerpt from Wikipedia:
The Hebrew word נָחָשׁ (Nachash) is used to identify the serpent that appears in Genesis 3:1, in the Garden of Eden. In Genesis, the serpent is portrayed as a deceptive creature or trickster, who promotes as good what God had forbidden and shows particular cunning in its deception. (cf. Gen. 3:4–5 and 3:22) The serpent has the ability to speak and to reason: "Now the serpent was more subtle (also translated as "cunning") than any beast of the field which the Lord God had made" (Gen. 3:1).
In the film, the couple abducted children as part of their "war on God". This would make the parents of the abducted children "demons" as they desperately search for their kids. Dover, in a way, does turn into a demon as he tortures an innocent, mentally challenged Alex (getting his own faith challenged in the process).
The snake symbolizes chaos, a chaos that turns good people to doing things they aren't supposed to (eating the apple despite strict instructions from God). In this way the snake embodies the chaos the Jones' want to unleash on their victims. In their view it's anti-Christian.
There is no direct reference to this in the film but I'm pretty sure, given the religious symbolism in the film, this is the explanation for the snakes.