What do they say? That the kid's father is a religious activist?
It seems that pretty much anybody with religion can be described as as a "religious activist". After all, what makes a successful one is for the ability of the followers to spread the word...
They all have youth indoctrination schemes (a.k.a Sunday school, boys brigade, after school programs), praying on those in need (a.k.a. city missions, soup kitchens, orphanages), condone the intimidation of others in a public place (a.k.a. street preaching). members are actively involved in financially supporting the organisation (attend church fairs).
Most even have regular meetings to reinforce their beliefs and hopefully inspire it in others, and get upset if somebody gets overly friendly with those from outside of their faith. They even force their faith on their children - think of the children!
and to top if off (OMG!) some of them even call themselves an army, with uniforms, ranks and everything... What if he was Christian and a member of "The Salvation Army"?
As far as I can tell holding religious ideas and beliefs are not illegal in Texas - but it is the breaking of laws that is. His Dad can think whatever he wants as long as his deeds are lawful.