Meta. Divine sonship. Isaac, meaning laughter, signifies the joy of the new birth and the new life in Christ, which is the spiritual consciousness of relationship to God the Father. Man rejoices greatly in his privilege of expressing as the son of God.
According to the text, Isaac was born after Sarah was past the age for bringing forth. Besides, she was barren, so that there was no possibility of his conception under the natural course of things. So we, when born of Spirit, are born, "not... of the will of the flesh, nor of the will of man, but of God." The natural man has no power to bring forth "the new man" in Christ Jesus. So Hagar's son could not be the chosen seed and heir. The new man is a "new creature," begotten by the divine seed, the Word. An entirely new state of consciousness is formed, fulfilling the admonition, let "Christ be formed in you."
When Isaac was weaned, Ishmael, Hagar's son, mocked him. This is the experience of every one in the new birth. The thoughts that are the fruit of the "mind of the flesh" rise up within and mock the new man. Here the overcomer has a work to do. Hagar, the bondmaid and her son must be cast out. Abraham grieves at this. So we sometimes grieve over giving up the fruits that we have brought forth in the natural-man consciousness.