Yes its true:
Jeremiah 17
9 The heart is deceitful above all things, and desperately wicked: who can know it?
10 I the LORD search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings.
Notice it's the human heart, not just Jeremiah's or the nation of Israel.
It's not only deceitful but it's deceitful above all things.
It's not only wicked but it's wicked to the point of desparation.
No one can know it, not even of their own heart without a direct revelation from God the Holy Spirit (conviction of sin) who can be grieved and quenched.
As the day grows darker and the sunset of man's reign upon earth approaches, the restraint of the Spirit will recede from the earth and humanity will revert to it's unbridled nature as stated in Jeremiah 17:9-10.
The previous sentence is my summary view of:
Luke 21
26 Men's hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken.
27 And then shall they see the Son of man coming in a cloud with power and great glory.
28 And when these things begin to come to pass, then look up, and lift up your heads; for your redemption draweth nigh.
2 Thessalonians 2
3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
2 Thessalonians 2:7 For the mystery of iniquity doth already work: only he who now letteth
will let, until he be taken out of the way.
HankD