What is the Oldest Truth in the Bible?

It is this: What we reproduce out of us is what we receive into us.

The answer to the question in the title is in Genesis 1: “Things produce after their own kind.” It is repeated 5–6 times. It’s another way of saying, “What is inside of us comes out of us.” Thus we’re considered to be ‘inside-out’ creations. So psychology says we can’t consistently act, on the outside, different from what we think and believe on the inside. This is a very important truth: It’s the very first thing mentioned by God.