‘That you may prove what is that good’ simply means describing the result or expectation of being renewed in your mind. It is proving or testing what God’s will is with the prospect of putting it into practice. We are to prove by practicing what pleases God.Ephesians 5:8-10 mentions that “For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light: (For the fruit of the Spirit is in all goodness and righteousness and truth ;) Proving what is acceptable unto the Lord.”We must be different from the people of the world by showing casing God’s righteousness in us.