In the lines that follow a righteous person is described. Who can live on God's holy mountain? The one who lives blamelessly and practices righteousness. A person who speaks the truth and does no harm to his or her neighbor. The one who keeps their word and does not take a bribe against the innocent. Psalm 15 is a song about holy, righteous living and how such a life keeps us in proximity to the Lord.
A bad contrast is often made between the Old and the New Testaments. In this view, the Old Testament describes a religion of "works," and the New Testament a religion of "grace." Martin Luther did a lot to create this distorted contrast and implant it in the Protestant mind. Read properly, however, there is no conflict between the Old and New Testaments on the central role of holy and righteous action in the world.
This might seem to be a moralizing conclusion, but the Christian monastic tradition helps me here. Holiness isn't moral striving, forcing yourself to "be a better person." Holiness is, rather, a lens. A window.
As Jesus said, the pure in heart will see God.
