Psalm 12:2

Everyone says false words to his neighbour: their tongues are smooth in their talk, and their hearts are full of deceit.

Psalm 41:6

If one comes to see me, deceit is in his heart; he keeps a store of evil, which he makes public in every place.

Romans 16:18

For such people are not servants of the Lord Christ, but of their stomachs; and by their smooth and well-said words the hearts of those who have no knowledge of evil are tricked.

Psalm 10:7

His mouth is full of cursing and deceit and false words: under his tongue are evil purposes and dark thoughts.

Jeremiah 9:8

His tongue is an arrow causing death; the words of his mouth are deceit: he says words of peace to his neighbour, but in his heart he is waiting secretly for him.

1 Chronicles 12:33

Of Zebulun, there were fifty thousand men, who went out with the army, expert in ordering the fight, to give help with all sorts of arms; true-hearted men.

Psalm 5:9

For no faith may be put in their words; their inner part is nothing but evil; their throat is like an open place for the dead; smooth are the words of their tongues.

Psalm 28:3

Do not take me away with the sinners and the workers of evil, who say words of peace to their neighbours, but evil is in their hearts.

James 1:8

For there is a division in his mind, and he is uncertain in all his ways.

Psalm 36:3-4

In the words of his mouth are evil and deceit; he has given up being wise and doing good.

Psalm 52:1-4

Why do you take pride in wrongdoing, lifting yourself up against the upright man all the day?

Psalm 55:21

The words of his mouth were smoother than butter, but war was in his heart; his words were softer than oil, but they were sharp swords.

Psalm 59:12

Because of the sin of their mouths and the word of their lips, let them even be taken in their pride; and for their curses and their deceit,

Psalm 62:4

Their only thought is to put him down from his place of honour; their delight is in deceit: blessing is in their mouths but cursing in their hearts. (Selah.)

Proverbs 20:19

He who goes about talking of the business of others gives away secrets: so have nothing to do with him whose lips are open wide.

Proverbs 29:5

A man who says smooth things to his neighbour is stretching out a net for his steps.

Jeremiah 9:2-6

If only I had in the waste land a night's resting-place for travellers, so that I might go away, far from my people! for they are all untrue, a band of false men.

Ezekiel 12:24

For there will be no more false visions or smooth use of secret arts in Israel.

1 Thessalonians 2:5

For it is common knowledge among you that we never made use of smooth-sounding false words, and God is witness that at no time were we secretly desiring profit for ourselves,

Treasury of Scripture Knowledge

Summary

They speak vanity every one with his neighbour: with flattering lips and with a double heart do they speak.

A double heart

Bible References

Flattering

Psalm 5:9
For no faith may be put in their words; their inner part is nothing but evil; their throat is like an open place for the dead; smooth are the words of their tongues.
Psalm 28:3
Do not take me away with the sinners and the workers of evil, who say words of peace to their neighbours, but evil is in their hearts.
Psalm 62:4
Their only thought is to put him down from his place of honour; their delight is in deceit: blessing is in their mouths but cursing in their hearts. (Selah.)
Proverbs 20:19
He who goes about talking of the business of others gives away secrets: so have nothing to do with him whose lips are open wide.
Proverbs 29:5
A man who says smooth things to his neighbour is stretching out a net for his steps.
Ezekiel 12:24
For there will be no more false visions or smooth use of secret arts in Israel.
Romans 16:18
For such people are not servants of the Lord Christ, but of their stomachs; and by their smooth and well-said words the hearts of those who have no knowledge of evil are tricked.
1 Thessalonians 2:5
For it is common knowledge among you that we never made use of smooth-sounding false words, and God is witness that at no time were we secretly desiring profit for ourselves,

A double heart

1 Chronicles 12:33
Of Zebulun, there were fifty thousand men, who went out with the army, expert in ordering the fight, to give help with all sorts of arms; true-hearted men.

General references

Genesis 29:19
And Laban said, It is better for you to have her than another man: go on living here with me.
Judges 16:6
So Delilah said to Samson, Make clear to me now what is the secret of your great strength, and how you may be put in bands and made feeble.
Habakkuk 1:3
Why do you make me see evil-doing, and why are my eyes fixed on wrong? for wasting and violent acts are before me: and there is fighting and bitter argument.
Acts 12:22
And the people, with loud cries, said, It is the voice of a god, not of a man.

Basic English, produced by Mr C. K. Ogden of the Orthological Institute - public domain