David says in this verse that he has done what he intended to do in Psalm 145:1: draw the line to the King. This psalm sings about God’s kingship, and therefore Christians can enthusiastically sing this psalm. While singing, Christ the King will be on your mind. At the same time, every Christian knows that the full riches of that kingship is still to come. Christ has taught us that we are to pray for that—Matthew 6:10: Your kingdom come.
That day will surely come—1 Timothy 6:14–15: until the appearing of our Lord Jesus Christ.
The Gospels very often talk about Jesus Christ as King. In him, this psalm finds its fulfillment. That means that ultimately Psalm 145 is about him, and in him it becomes completely true. We also see other references to Jesus that can be connected to this Psalm e.g., Revelation 15:3 and Revelation 17:14.
In Psalm 145, the generations are called upon to proclaim the mighty deeds of the Lord and to sing of them. The current generation must also do that! The status of the New Testament generation and the mission beautifully converge in 1 Peter 2:9 (see also 1 Peter 2:6 and 1 Peter 2:12). And the reason for the praise is—as it says there—the redemption from darkness by Christ.
21 My mouth will speak the praise of the LORD, and let all flesh bless his holy name forever and ever.