John Calvin Commentary Genesis 26:5

"Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws." — Genesis 26:5 (ASV)

Because that Abraham obeyed my voice. Moses does not mean that Abraham’s obedience was the reason why the promise of God was confirmed and ratified to him. Rather, from what has been said before (Genesis 22:18), where we have a similar expression, we learn that what God freely bestows upon the faithful is sometimes, undeservedly, attributed to them, so that they, knowing their intention is approved by the Lord, may more ardently dedicate and devote themselves entirely to His service. Therefore, He now commends Abraham’s obedience so that Isaac may be stimulated to imitate his example.

Although laws, statutes, rites, precepts, and ceremonies had not yet been written, Moses used these terms to show more clearly how diligently Abraham regulated his life according to the will of God alone—how carefully he abstained from all the impurities of the heathen, and how exactly he pursued the straight course of holiness, without turning aside to the right hand or to the left. For the Lord often honors His own law with these titles to restrain our excesses, as if to say that it lacked nothing to be a perfect rule but embraced everything pertaining to absolute holiness.

The meaning, therefore, is that Abraham, having formed his life in complete accordance with the will of God, walked in His pure service.