Those rules depend on which denomination you’re asking. The pre-schism church decided you didn’t need to be Christian at all, or even monotheistic. They basically decided that rejecting god was actually a high bar (hearing about Christianity and not joining wasn’t even close, and even being educated as a Christian and not believing anyway wasn’t enough) and that people who were righteous would get in regardless of faith. After all, it’s not your fault if you were raised in a distant land where Christianity wasn’t a thing, and you couldn’t really be blamed for not believing without a visitation from god himself, or an angel, or witnessing a miracle, or similar. Of course, it’s easier to be righteous if you’re Christian and know the rules, so spreading the faith was important. This is even still the official stance of the Vatican (and, I would assume, most Orthodox churches). A lot of the black and white morality and biblical literalism comes from Protestant factions. Definitely not all of it, though; try asking the Pope what he thinks about divorce or gay marriage.