Filled with joy are those who are attacked because they follow Jesus.
Filled with joy are those who are mocked and scorned and lied about because they obey Me.
Let your joy be known and overflow because in heaven there is a great reward for you, because you are following in the footsteps of great followers of Jesus. (Matthew 5:10-12, my imperfect paraphrase).
Why would I be filled with joy? What reward?
The reward is that I will experience a love for my attacker that transcends human logic, transcends human wisdom, and transcends “earthly” love. And my attacker will “see” the Jesus they say does not exist and have a chance to choose to know Him for themselves.