In Genesis 15.4 God said to Abram, “he that shall come forth out of thine own bowels shall be thine heir.” Abram and Sarai, because they were married, were as one. The promised heir should have been born of Sarai. However, as time passed and it seemed that God would not fulfill His promise, Sarai devised her own plan, to which Abram consented. The plan was to have a child by one who was not Abram’s wife (flesh) - Sarai’s maid, Hagar. In so doing, they did not rely upon the promise of God.