AT fields are a metaphor for the emotional barriers we put up between ourselves and others. Barriers that help us feel safe but also prevent us from achieving true intimacy. This ties in to Shinji's arc. There is even an episode of the original series titled "Hedgehogs Dilemma" which is what some philosopher called this dilemma. What the AT fields actually "are" is never really explained but it's also irrelevant to what the show is trying to do and say.

Shakespeare would still be Shakespeare even if it was written by a trillion monkeys smashing keys on typewriters.

It isn't about Christianity.

To be honest the movies barely make any sense at all if you have not watched the original series.

Yeah, but there is meaning in stylization. Anno picked the religious iconography because it matched the vibe he was trying to create. He understood how including that imagery would affect the audience emotionally. Religion is the tool that some people use to answer the philosophical questions posed by the show, so including that imagery puts the audience in the right mindset.

It's not about religion