Treasury of Scripture Knowledge
Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
For what is known about God is plain to these people [i.e., to the Gentiles], because God has made it clear to them.
Now this is [how God's] judgment [works]: The Light [See 1:5-9] has come into the world, but people loved darkness [i.e., sin] rather than the Light, because their deeds were evil.
and so that the Gentiles [also] may honor God for His mercy, as it is written [Psa. 18:49], "Therefore, I will praise you among the Gentiles and sing to your name." [Note: Paul here accommodates the words of King David by applying them to Jews participating with Gentiles in praising God as part of the one body].
Then one of them, seeing he had been healed, returned and praised God in a loud voice.
2 Timothy 3:2
For people will be lovers of themselves, lovers of money, boastful, arrogant, abusive talkers, disobedient to their parents, ungrateful, unholy,
And he said [to them] in a loud voice, "Fear God and honor Him, for the time has come for Him to judge people. Worship Him who made the heaven, the earth, the ocean and the springs of water."
O Lord, who will not fear [you] and honor your name, for only you are holy? [So], all the nations will come and worship in front of you, for your righteous deeds have been revealed."
So, I tell you this, and insist [on it] by [the authority of] the Lord: Do not continue to live with worthless thoughts as [unconverted] Gentiles do.
1 Peter 1:18
You [should] know that you were bought back [from Satan], from your useless way of life, inherited from your forefathers, not with such perishable things as silver and gold,
Let their eyes become [spiritually] dim so they cannot see, and may their backs always bend [i.e., under heavy burdens]."
to open their eyes [to the truth] so they will turn from darkness to light and from the power [i.e., the influence] of Satan to God. [Then] they can receive the forgiveness of sins and the inheritance [of never ending life], along with [all] those who are set apart [for God] by trusting in Me.'
1 Peter 2:9
But you people are especially chosen [by God]; you are a royal body of priests; you are a nation especially set apart for God's use; you are a people who belong exclusively to God, that you may express the virtues of the One who called you out of darkness [i.e., of sin and error], and into His marvelous light [i.e., righteousness and truth].
So, the person who knows the good [he ought] to be doing, but is not doing it, is sinning.