You are witnesses with God, how holy, righteously, and blamelessly we behaved ourselves toward you who believe.
how holily and justly and unblameably we behaved ourselves: Paul uses three adverbs to describe his behavior. Sharp distinctions in meaning may not be intended. However, "holily" probably refers to the fact that Paul has been "set apart" for divine service; "justly" seems to intimate that the apostles are in conformity to a norm, the revealed will of God; and "unblameably" suggests their conduct is of such high moral quality that no one could justifiably accuse them of any breach of God’s standard of morality.