For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. His son by the slave woman was born in the ordinary way; but his son by the free woman was born as the result of a promise. (Galatians 4:22-23, NIV)
How can Abraham, the best example of faith among the patriarchs, also be an example of faithless human effort? Abraham was the one who did something quite unheard of in his day. He left his country and the land where his father's family lived to go where God would show him (Genesis 12:1-5). We want God to tell us His will specifically before we start to do it. Abraham did better when he departed not knowing the destination until God told him that he was there Genesis 12:6-8.
When God gave Abraham the specific promise of a son (Genesis 15:1-6), a promise that was humanly impossible, Abraham and Sarah spent their time figuring out how God would do it. Sarah and Abraham used the common practice of their day. Sarah used her maidservant as a surrogate mother for Abraham's child (Genesis 16:1-3), a child that was legally also Sarah's child. However, this child was a child from the human effort of Abraham and Sarah to accomplish God's promise through their own means. Ishmael was not the child God promised. God did what was unimaginable to Abraham and Sarah (Genesis 18:9-15). When Sarah was too old to bear children, she conceived and bore their son, Isaac (Genesis 21:1-7).
How often do we feel the urge to help God out in accomplishing His will in our lives? Undesirably, this temptation leads us to use all the human means available to us, even means acceptable by the human standards of our day, but not to God. Living by faith is living by what God has told us is right in the scriptures and trusting God to work out the circumstances of His will.