In Psalm 1:1-3 we have been looking at the godly man.  Verses 4-5 now speak of the ungodly man.
“The wicked are not so,
 But they are like chaff which the wind drives away.
 Therefore, the wicked will not stand in the judgment 
 Nor sinners in the assembly of the righteous.”

The godly man is like a tree, but the ungodly is like chaff.  Notice the contrast is not tree to tree, for the tree is alive and rooted; but the chaff is dead and blown away.  This I think gives a very clear picture that man is either one or the other, there is no middle ground.  For Christ died that we might have life and all who receive Him are born anew;
“Therefore, if any man is in Christ, he is a new creation…” [2 Corinthian 5:17]
Those who receive Him not are still dead in their trespasses and sin.

From “Vine’s Expository Dictionary”:
ACHURON is the Greek word for chaff and is defined as: ‘the stalk of the grain from which the  kernels have been beaten out, or the straw broken up by a threshing machine.’  
‘Like chaff which is blown away…the wicked will not stand in the judgment,’ nor will sinners be included with the righteous.
Scripture tells us what happens to the chaff and likewise to the ungodly man:
“Therefore, as the fire devours the stubble,
 And the flame consumes the chaff, 
 So their root will be as rottenness,
 And their blossom will ascend like dust; 
 Because they have rejected the law of the LORD of hosts,
 And despised the word of the Holy One of Israel.”  Isaiah 5:24

They are, 
“…Like chaff blown off from a threshing floor
 And like smoke from a chimney.”  Hosea 13:3

John the Baptist tells us in Luke 3:17,
“His (Jesus) winnowing fan is in His hand and He will thoroughly purge His threshing floor, and gather the wheat into His barn; but the chaff He will burn with unquenchable fire.”   (My parenthesis)

The wicked were warned:
“Were they ashamed when they had committed abomination?
 No! They were not ashamed;
 Nor did they know how to blush.
 Therefore they shall fall among those who fall…”  Jeremiah 6:15

They did not delight in nor meditate on the law of the LORD or His commandments, but despised and rejected them; they did not honor God.

“…The wicked will fall by his own wickedness.”  Proverbs 11:5

There is no secure footing for the ungodly!
“Therefore their way shall be to them like slippery ways;
 In the darkness they shall be driven on and fall in them.”  Jeremiah 23:12

Then Jesus in Matthew: Vs. 26-27 tells us,
“Now everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand;
And the rain descended, the floods came and the winds blew and beat on that house, and it fell.  And great was the fall,”

Then comes the judgment:
“…the Lord knows how to deliver the godly out of temptation and to reserve the unjust under punishment for the day of judgment.”  2 Peter 2:9

A final separation:
“So it will be at the end of the age.  The angels will come forth, separate the  wicked from among the just, and cast them into the furnace of fire.  
There will be wailing and the gnashing of teeth.”  Matthew 13:49-50

The last verse of Psalm 1 states:
“For the LORD knows the way of the righteous,
 But the way of the wicked will perish.”  Psalm 1:6

Rejoice that our Lord and Savior prepared our hearts to say yes to the One who gives life eternal.  It is by God’s grace alone that we are saved.  With grateful hearts we say, “THANK YOU!”
Blessed be our God forever and forever more.  Amen