Psalm 149 seems to be two sides of a coin. It starts out with a command for the saints to sing and praise God their maker and savior and then in verse 6 introduces the idea of a double edged sword in their hands to inflict vengeance on the nations. But the Bible says, "Vengeance is Mine says the Lord." Revelation 19:15 talks about a sharp sword coming from Jesus' mouth to strike down the nations and calls it a day of vengeance. Could this be God's word that either convicts the heart or hardens the heart and is thereby that by which they are judged? Try saying that five times fast!
So to summarize by plain old John (me), we are to praise God for His goodness and to spread His word about His goodness so that others can make that decision to accept or reject their Maker and would be Savior. John 1:1 and 1 John 1:1 both describe Jesus as the Word. So, again, it sounds to me that praising God and singing a new song about Him is kind of the same thing as the double edged sword in their hands. We are to present Jesus to those around us by our words and actions in a way that demonstrates His character. And, of course, we can only do that through His indwelling Spirit. Also see Proverbs 5:4.
Proverbs 4
Security in Wisdom
1 Hear, my children, the instruction of a father,
And give attention to know understanding;
2 For I give you good doctrine:
Do not forsake my law.
3 When I was my father’s son,
Tender and the only one in the sight of my mother,
4 He also taught me, and said to me:
“Let your heart retain my words;
Keep my commands, and live.
5 Get wisdom! Get understanding!
Do not forget, nor turn away from the words of my mouth.
6 Do not forsake her, and she will preserve you;
Love her, and she will keep you.
7 Wisdom is the principal thing;
Therefore get wisdom.
And in all your getting, get understanding.
8 Exalt her, and she will promote you;
She will bring you honor, when you embrace her.
9 She will place on your head an ornament of grace;
A crown of glory she will deliver to you.”
10 Hear, my son, and receive my sayings,
And the years of your life will be many.
11 I have taught you in the way of wisdom;
I have led you in right paths.
12 When you walk, your steps will not be hindered,
And when you run, you will not stumble.
13 Take firm hold of instruction, do not let go;
Keep her, for she is your life.
14 Do not enter the path of the wicked,
And do not walk in the way of evil.
15 Avoid it, do not travel on it;
Turn away from it and pass on.
16 For they do not sleep unless they have done evil;
And their sleep is taken away unless they make someone fall.
17 For they eat the bread of wickedness,
And drink the wine of violence.
18 But the path of the just is like the shining sun,
That shines ever brighter unto the perfect day.
19 The way of the wicked is like darkness;
They do not know what makes them stumble.
20 My son, give attention to my words;
Incline your ear to my sayings.
21 Do not let them depart from your eyes;
Keep them in the midst of your heart;
22 For they are life to those who find them,
And health to all their flesh.
23 Keep your heart with all diligence,
For out of it spring the issues of life.
24 Put away from you a deceitful mouth,
And put perverse lips far from you.
25 Let your eyes look straight ahead,
And your eyelids look right before you.
26 Ponder the path of your feet,
And let all your ways be established.
27 Do not turn to the right or the left;
Remove your foot from evil.
Proverbs 5
The Peril of Adultery
1 My son, pay attention to my wisdom;
Lend your ear to my understanding,
2 That you may preserve discretion,
And your lips may keep knowledge.
3 For the lips of an immoral woman drip honey,
And her mouth is smoother than oil;
4 But in the end she is bitter as wormwood,
Sharp as a two-edged sword.
5 Her feet go down to death,
Her steps lay hold of hell.
6 Lest you ponder her path of life—
Her ways are unstable;
You do not know them.
7 Therefore hear me now, my children,
And do not depart from the words of my mouth.
8 Remove your way far from her,
And do not go near the door of her house,
9 Lest you give your honor to others,
And your years to the cruel one;
10 Lest aliens be filled with your wealth,
And your labors go to the house of a foreigner;
11 And you mourn at last,
When your flesh and your body are consumed,
12 And say:
“How I have hated instruction,
And my heart despised correction!
13 I have not obeyed the voice of my teachers,
Nor inclined my ear to those who instructed me!
14 I was on the verge of total ruin,
In the midst of the assembly and congregation.”
15 Drink water from your own cistern,
And running water from your own well.
16 Should your fountains be dispersed abroad,
Streams of water in the streets?
17 Let them be only your own,
And not for strangers with you.
18 Let your fountain be blessed,
And rejoice with the wife of your youth.
19 As a loving deer and a graceful doe,
Let her breasts satisfy you at all times;
And always be enraptured with her love.
20 For why should you, my son, be enraptured by an immoral woman,
And be embraced in the arms of a seductress?
21 For the ways of man are before the eyes of the Lord,
And He ponders all his paths.
22 His own iniquities entrap the wicked man,
And he is caught in the cords of his sin.
23 He shall die for lack of instruction,
And in the greatness of his folly he shall go astray.
Proverbs 6
Dangerous Promises
1 My son, if you become surety for your friend,
If you have shaken hands in pledge for a stranger,
2 You are snared by the words of your mouth;
You are taken by the words of your mouth.
3 So do this, my son, and deliver yourself;
For you have come into the hand of your friend:
Go and humble yourself;
Plead with your friend.
4 Give no sleep to your eyes,
Nor slumber to your eyelids.
5 Deliver yourself like a gazelle from the hand of the hunter,
And like a bird from the hand of the fowler.
The Folly of Indolence
6 Go to the ant, you sluggard!
Consider her ways and be wise,
7 Which, having no captain,
Overseer or ruler,
8 Provides her supplies in the summer,
And gathers her food in the harvest.
9 How long will you slumber, O sluggard?
When will you rise from your sleep?
10 A little sleep, a little slumber,
A little folding of the hands to sleep—
11 So shall your poverty come on you like a prowler,
And your need like an armed man.
The Wicked Man
12 A worthless person, a wicked man,
Walks with a perverse mouth;
13 He winks with his eyes,
He shuffles his feet,
He points with his fingers;
14 Perversity is in his heart,
He devises evil continually,
He sows discord.
15 Therefore his calamity shall come suddenly;
Suddenly he shall be broken without remedy.
16 These six things the Lord hates,
Yes, seven are an abomination to Him:
17 A proud look,
A lying tongue,
Hands that shed innocent blood,
18 A heart that devises wicked plans,
Feet that are swift in running to evil,
19 A false witness who speaks lies,
And one who sows discord among brethren.
Beware of Adultery
20 My son, keep your father’s command,
And do not forsake the law of your mother.
21 Bind them continually upon your heart;
Tie them around your neck.
22 When you roam, they will lead you;
When you sleep, they will keep you;
And when you awake, they will speak with you.
23 For the commandment is a lamp,
And the law a light;
Reproofs of instruction are the way of life,
24 To keep you from the evil woman,
From the flattering tongue of a seductress.
25 Do not lust after her beauty in your heart,
Nor let her allure you with her eyelids.
26 For by means of a harlot
A man is reduced to a crust of bread;
And an adulteress will prey upon his precious life.
27 Can a man take fire to his bosom,
And his clothes not be burned?
28 Can one walk on hot coals,
And his feet not be seared?
29 So is he who goes in to his neighbor’s wife;
Whoever touches her shall not be innocent.
30 People do not despise a thief
If he steals to satisfy himself when he is starving.
31 Yet when he is found, he must restore sevenfold;
He may have to give up all the substance of his house.
32 Whoever commits adultery with a woman lacks understanding;
He who does so destroys his own soul.
33 Wounds and dishonor he will get,
And his reproach will not be wiped away.
34 For jealousy is a husband’s fury;
Therefore he will not spare in the day of vengeance.
35 He will accept no recompense,
Nor will he be appeased though you give many gifts.
Psalm 149
Praise to God for His Salvation and Judgment
1 Praise the Lord!
Sing to the Lord a new song,
And His praise in the assembly of saints.
2 Let Israel rejoice in their Maker;
Let the children of Zion be joyful in their King.
3 Let them praise His name with the dance;
Let them sing praises to Him with the timbrel and harp.
4 For the Lord takes pleasure in His people;
He will beautify the humble with salvation.
5 Let the saints be joyful in glory;
Let them sing aloud on their beds.
6 Let the high praises of God be in their mouth,
And a two-edged sword in their hand,
7 To execute vengeance on the nations,
And punishments on the peoples;
8 To bind their kings with chains,
And their nobles with fetters of iron;
9 To execute on them the written judgment—
This honor have all His saints.
Praise the Lord!