@ScumbagDog Its hardly limited to religion. Most people decide their opinions first, usually based on who has the same opinion (a desire to fit in) or to coincide with other existing beliefs (like the world being 4000 years old). The next step is to find sources that agree with what they decided to was true and then pretend that they it is true because of those sources, so it sounds authoritative and not just pulled out of their ass.

The bible just happens to do a great job as one of those sources because it is vaguely worded and seemingly contradictory so it can be twisted to say almost anything they want it to say.