These verses appear to be an early Christian hymn that the author has included in his overall message.
The New Testament contains liturgical materials (creeds, confessions, and hymns) that existed prior to the books themselves. In the present instance a hymn concerning Christ is used to further what is being said about endurance in suffering for the gospel. The hymn speaks of Christ’s death, resurrection, and role in the final judgment. Most startling about it is its affirmation that Christ remains faithful, even if Christians fail to be faithful to him.