Paul, like the other biblical writers, when seeking to make a point, employ two or three witnesses from previous canonical writers to show the unity of the message given by God by the inspiration of the Spirit. This was in fact how the present canon was formed, by showing that new revelation was consistent with what had come before. In this passage, Paul’s main point is garnered from the three old testament references he refers to. Speaking truth to one’s neighbour is the opposite of lying, as Zechariah stated (8:16), which Paul applies to fellow members in the church (v. 25).
One can be angry without sin, and therefore we should not harbour unjustified sinful anger against anyone in the church, for to do so would be to give the devil a foothold (v. 26). These second references may come from two of the Psalms, 4:4 and 37:8. The antidotes, according to these references, is to meditate, ie., keep silent, practice righteousness to the Lord, and trust in the Lord, waiting patiently on the Lord to have his way. Fostering sinful anger gives a place for the devil to operate from within the body (v. 27). Stealing from one another is also forbidden, instead we should give to those in need, as we are able (v. 28).
There is a time to keep silent, but there is also a time to speak, as long as our words are edifying, imparting grace to those in need (v. 29 cf. Eccl. 3:7). Anyone not following these principles is in danger of grieving the Holy Spirit, by whom the saints are sealed for a future full redemption (v. 30). This is not to suppose that we can be perfect in this life. If this were the case then none could be saved, there would be then no need for these admonishments, and also no need for forgiveness, one to another (vv. 31-32). Verse 31 lists the sinful marks, which we must “put away,” evidencing instead the fruit of the Spirit (v. 32).