It was a promise that sounded like a joke. Abram (later to become Abraham), unable to produce an heir after decades with his wife Sarai (later Sarah), was told to count the stars – if he could. “That is how numerous your descendants will be” said God.
Let’s say that polite disbelief was a reasonable response! However, if there was a joke, the joke was on Abram when his dynasty began with a son, born when he and is wife were right at the century mark.
However, it is not the obstetrical miracle that was the real point of the story – it was the promise, unbelievable as it seemed at the time. This promise, this “you can believe it because I said it” covenant that God makes to Abram that is the start of the great theme of the Scriptures. God promise and provision begun in Genesis 15 finds its great culmination in the work of Christ for you and me.