Romans 1:21

because, having known God, they did not glorify Him as God, nor did they give thanks; but they became vain in their reasonings, and their senseless heart was darkened.

Ephesians 4:17-18

This, therefore, I say, and testify in the Lord, that ye no longer walk as the gentiles also walk, in the vanity of their mind,

Romans 1:19

because that which is known of God is manifested in them; for God manifested it to them.

Revelation 14:7

saying with a great voice, "Fear God, and give glory to Him, because the hour of His judgment came; and worship Him Who made the heaven, and the earth, and the sea, and fountains of waters."

Luke 17:15-18

And one of them, seeing that he was cleansed, turned back, praising God with a loud voice,

John 3:19

And this is the judgment, that the Light hath come into the world, and men loved the darkness rather than the Light; for their works were evil.

Romans 1:28

And, as they did not approve the holding of God in their knowledge, God delivered them up to a reprobate mind, to do those things which are not becoming;

Romans 11:10

let their eyes be darkened, that they may not see, and bow down their back always."

Romans 15:9

and that the gentiles might glorify God for His mercy; as it has been written, "For this cause, I will confess to Thee among the gentiles; and to Thy name will I sing praise."

2 Timothy 3:2

for men will be lovers of themselves, lovers of money, boasters, proud, blasphemers, disobedient to parents, ungrateful, unholy,

1 Peter 1:18

knowing that, not with perishable things, with silver or gold, were ye redeemed from your vain course of life handed down from your fathers;

1 Peter 2:9

But ye are "an elect race, a kingly priesthood, a holy nation," a people for a possession, that ye may show forth the excellencies of Him Who called you out of darkness into His marvelous light;

Revelation 15:4

Who will not fear, O Lord, and glorify Thy name? because Thou only art holy; because all the nations shall come, and worship before Thee; because Thy righteous acts were made manifest."

Acts 26:18

to open their eyes, that they may turn from darkness to light, and from the dominion of Satan to God, that they may receive remission of sins, and an inheritance among those who have been sanctified by faith in Me.'

Treasury of Scripture Knowledge

Summary

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.

General references

Bible References

When

Romans 1:19
because that which is known of God is manifested in them; for God manifested it to them.
John 3:19
And this is the judgment, that the Light hath come into the world, and men loved the darkness rather than the Light; for their works were evil.

They glorified

Romans 15:9
and that the gentiles might glorify God for His mercy; as it has been written, "For this cause, I will confess to Thee among the gentiles; and to Thy name will I sing praise."
Luke 17:15
And one of them, seeing that he was cleansed, turned back, praising God with a loud voice,
2 Timothy 3:2
for men will be lovers of themselves, lovers of money, boasters, proud, blasphemers, disobedient to parents, ungrateful, unholy,
Revelation 14:7
saying with a great voice, "Fear God, and give glory to Him, because the hour of His judgment came; and worship Him Who made the heaven, and the earth, and the sea, and fountains of waters."
Revelation 15:4
Who will not fear, O Lord, and glorify Thy name? because Thou only art holy; because all the nations shall come, and worship before Thee; because Thy righteous acts were made manifest."

But became

Ephesians 4:17
This, therefore, I say, and testify in the Lord, that ye no longer walk as the gentiles also walk, in the vanity of their mind,
1 Peter 1:18
knowing that, not with perishable things, with silver or gold, were ye redeemed from your vain course of life handed down from your fathers;

Their foolish

Romans 11:10
let their eyes be darkened, that they may not see, and bow down their back always."
Acts 26:18
to open their eyes, that they may turn from darkness to light, and from the dominion of Satan to God, that they may receive remission of sins, and an inheritance among those who have been sanctified by faith in Me.'
1 Peter 2:9
But ye are "an elect race, a kingly priesthood, a holy nation," a people for a possession, that ye may show forth the excellencies of Him Who called you out of darkness into His marvelous light;

General references

James 4:17
To him, therefore, who knows to do good, and does it not, to him it is sin.