A very basic lesson we need to understand is the importance of giving God a chance to fulfill a promise. And so, we have the story of Isaac and Ishmael. This is not a case of waiting for “God’s time”, but rather the fullness of time.
God promised Abraham that his wife Sarah would give birth to a son who would be named Isaac. But Sarah did not believe she could have a child at her age, and so he let Abraham use her servant Hagar as a surrogate. Hagar gave birth to Ishmael, who was given a different inheritance than the one promised to Isaac.