# in the word of truth

"by speaking God's message about truth" or "by speaking God's true message"

# in the power of God

"by showing God's power to people"

# with the armor of righteousness for the right hand and the left

Paul speaks of their righteousness as if it were weapons that they use to fight spiritual battles. 

# the armor of righteousness

"righteousness as our armor" or "righteousness as our weapons"

# for the right hand and the left

Possible meanings are 1) that there is a weapon in one hand and a shield in the other or 2) that they are completely equipped for battle, able to fend off attacks from any direction.