The shooter same to the US in 1996, at something like age 5. Being black and coming to the US at that time - when the Arab Muslim north was conducting a war of genocide against the Christian and animist blacks of the south - there's a good chance his family was Christian, animist, or nothing in particular.
Articles I've seen mention that the guy had attended that church some a year or two previous, but had not been there since. His FB page featured motivational comments suggesting one could create one's own reality and body-building or workout pix of himself.
Speculating, order of what seems likely to me at this point:
* The guy was on a I-am-my-own-god trip and became hatefully obsessed with that church, possibly with some influence from steroids which are used by some body-builders and also affect one's personality significantly ('roid rage may be a trite stereotype but there is some reality to it);
* The guy dove into the BLM type racial identity sewer when he was in college and figured the church was filled with easy white targets;
* The guy was either raised Muslim or converted in college and was "radicalized".
With OhBummer! out of office and diminished Federal pressure to avoid the M-word and certain other types of motivations, we may learn what the shooter was into.