Why did Isaac enjoy the favor of God? Was it because of his faith? He was a man of faith, to be sure (see Hebrews 11:20), but why did he enjoy God’s blessings? The book of Genesis offers two reasons why.
Sojourn in this land, and I will be with you and bless you; for to you and your descendants I will give all these lands, in fulfillment of the oath that I swore to your father Abraham. I will make your descendants as numerous as the stars in the sky, and I will give them all these lands, and in your descendants all the nations of the earth will find blessing—this because Abraham obeyed me, keeping my mandate, my commandments, my ordinances, and my instructions. (Genesis 26:3-5)
The first reason Isaac enjoyed God’s blessings is that he did what God told him to do: he stayed in the land, and in accordance with His promise God was with him and blessed him. But this is a condition of obedience, not of faith. Surely Isaac had faith, as I already mentioned. But that is not the reason that God promised to bless him. God promised to bless Isaac for his obedience in staying in the Promised Land. Faith and obedience go together.
The second reason Isaac enjoyed God’s blessings is that his father Abraham obeyed God. See the passage from Genesis 26 above: “Because Abraham obeyed me, keeping my mandate, my commandments, my ordinances, and my instructions.” Isaac’s descendants would be as numerous as the stars, and would inherit the Promised Land, and (referring specifically to the Lord Jesus: see Galatians 3:16) would be a blessing to all the nations of the earth not because of anything that Isaac did or believed, and not because of anything that Abraham believed, but rather because Abraham was obedient. These blessings were the fruit of Abraham’s faithful obedience to God. So we see that not only does obedience matter, but that it also merits God’s blessings after our deaths—blessings that God extends not just to us but also to others.
The point, of course, is that faith and obedience are inseparable.
[Update, 3 April 2016]
Whoever believes in the Son has eternal life, but whoever disobeys the Son will not see life, but the wrath of God remains upon him. (John 3:36)
In this verse the Lord Jesus makes pretty transparent the fact that obedience and faith go together. It is not enough to simply believe; if one disobeys Jesus, then “the wrath of God remains upon him.” Faith by itself is not a sufficient condition; faith and obedience are inseparable.