He prays that they will do no evil. In other words that they will be very concerned for personal righteousness and for the protection of the word of God.
This confidence that the believer has of the Lord’s favour is his refuge and something which he guards very closely. In order to maintain this confidence, the believer is careful to live according to his conscience, regardless of how he is judged by men. The conscience is able to give a strength to the soul that can bear great trials for the Lord.