In other words, Paul does not want his departing to have an effect upon their holiness, or the godliness of their church. Paul is pleading that they would present a consistent witness to the world, regardless of where he is. Many times, when the leader of a successful church departs, there is a fear that the church will fall apart. If the church is truly successful in the eyes of God, this should not be a concern because a truly successful church is under the leadership of Christ, in the guidance of the Holy Spirit. Many differences and petty quarrels are ignored, overlooked, and quickly forgiven when fighting under the same flag against a common enemy.
Unity is crucial to the church, which is in the midst of a spiritual war.
Unity is a crucial element to testify to the world that Christ is the Son of God. May they also be in us so that the world may believe that You have sent me. This takes effort. Pray fervently for their spritual struggles, mourn over their setbacks, stand with them in the battle, lending any aid that you can. To be frightened is to demonstrate a lack of faith in the cause of the battle, the strength of the army, and the wisdom of the general.