All along in Chapter 5 Paul has been speaking of imitating Christ and obviously not living like the world around the believer. What was the believer's past should be left there and what, or Who, is now in the believer should be the controlling factor in the believer's behavior. Paul said that Jesus, the Light of the World, now lives in their lives and they should live as "people of the light."
Now, in this verse Paul is telling the believer what to expect from the indwelling Light. Paul uses three terms to describe this expected behavior: First, GOOD - the very identity of God. Remember when the rich young ruler came to Jesus and called Him, "Good Master?" Remember how Jesus responded by saying "...only God is good." What He meant was telling this young man, "If you can't call me God, don't call me good." But God is Good! That's the natural behavior of our Creator. He is a good God and all that He does is ultimately just that. Romans 8:28 tells us to expect "good" out of every circumstance and situation in which we may find ourselves.
The second term is RIGHT. God is good in what He does and He is right in who He is! God is a righteous God and always acts in perfect righteousness. And that is what should be expected to be true of a Christian. They will live righteous lives in the midst of a wicked world. "Righteousness exalts a nation..." Proverbs 14:34. God is honored when people live as He does. But this righteousness can only come from the indwelling Christ. Righteousness apart from Christ is self-serving and is "filthy" in God's sight, Isaiah 64:6.
The third term is TRUE. Jesus said He is "the truth..." John 14:6. He always speaks and lives truth before a deceptive and lying world. He spoke the truth about God and about man, about man's failure and God's salvation. The Christian will always seek to speak the truth, but will also seek to live the truth.