Psalm 144: Only God

Verse 15: 

Blessed are the people of whom this is true; blessed are the people whose God is the Lord.

The specific prayers David made for his nation are a list of blessings for the nation and its people. The only condition David presents for this is in verse 15. 'Serving the Lord as their God' is an attitude of considering God, whether in joy or sorrow, living righteously or even sinning, seeing light during their lifetime or ending their life in darkness. Such people have no one else to rely on but God. Jesus spoke of this state with an appropriate parable. In Luke 18, the unjust widow had no one to appeal to but the crooked judge. The parable pays more attention to the widow's situation than the outcome. The reason why David's prayer for the nation is so heartfelt is because of verse 15, which he leaves like a last will. It talks about a nation and people who have no one else to serve but God.