There've always been religious crazies but they were mostly relegated to the fringes until the 50s when they needed a moral justification for the Cold War. We had just come out of WWII which had clear moral framing and needed similar to keep things going against the Soviets, so used Christianity as the moral high ground to spit down on the godless communists. When was the 'under God' added to the pledge that started this conversation? 1954. The pledge had been around for over fifty years before that. Then when the counterculture revolution happened in the 60s leading to the intellectual freedom of the 70s public consciousness fractured, causing the conservative backlash of the 80s when Reagan married the traditionally adversarial protestants and Catholics under the banner of white Christians vs. everybody else. And we've been in that world ever since. That majority continues to dwindle though as America becomes more diverse, so we're currently in the middle of another realignment as the Reaganite Republican Party loses influence and the Trump Republican Party rises. It's unlikely Christianity has the raw numbers for the consistent political majority that Reagan produced, as that'd require drafting most of the religious minority populations that have shunned that traditionally white coalition.
