I love this church. In the midst of hard times their faith is growing and their love is increasing. In order to encourage this church, Paul reminds them that justice will be exercised on those who are persecuting them on the final day. Faith that God will make things right is one key to enduring in the face of abuse.
The description of unbelief is especially telling -- They do not know God and they do not OBEY the gospel. This is more true and helpful than they way the gospel is commonly thought of, as an optional add-on to an already full life. It demands to be obeyed. To disobey it is to encounter the terrifying and eternal wrath of God.
The return of Christ will be terrifying and marvelous at the same time, depending on your perspective. The perspective to have, of course, is the perspective of faith. Then, Christ's return will be marvelous and glorious!