"I will greatly rejoice in the LORD, my soul shall be joyful in my God; for he hath clothed me with the garments of salvation, he hath covered me with the robe of righteousness, as a bridegroom decketh himself with ornaments, and as a bride adorneth herself with her jewels." (Isaiah 61: 10)
The prophet declares what makes us joyful.
Not winning the lottery, not getting the high school sweet-heart, and not having the admiration of others.
What gives us joy? "He has clothed me with the garments of salvation".
What is connected with this covering of salvation?
"The robe of righteousness".
Why do we have these clothes today?
Because of Jesus. First, He declared the necessity of God's righteousness:
"But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you." (Matthew 6: 33)
"Fear not, little flock; for it is your Father's good pleasure to give you the kingdom." (Luk3 12: 32)
We have this gift of righteousness today because of the grace of God, showed then showered on us at the Cross:
"For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ." (Romans 5: 17)
"For the kingdom of God is not meat and drink; but righteousness, and peace, and joy in the Holy Ghost." (Romans 14: 17)
Notice that joy, one of the fruits of the Holy Spirit (Galatians 5: 22-23), follows peace, which we have because we have been justified, made righteous, by faith in Jesus' Finished Work at the Cross (Romans 5: 1)
Righteousness, His righteousness, indeed makes us joyful, for we get to grow in grace and see what a great Work Jesus has wrought -- and continues to work -- for us through the Cross and His Ministry forever at the Father's right hand.