25Therefore, having put away falsehood, let each one of you speak the truth with his neighbor, for we are members one of another. 26Be angry and do not sin; do not let the sun go down on your anger, 27and give no opportunity to the devil.
Therefore, refers to previous verses that talk about no longer thinking in our old, futile ways. We have been renewed, so keep growoing in it! As a result of being renewed, we do not lie to one another, but we speak the truth!
Here is a bell-ringer: Sometimes when you speak the truth, people get angry! The truth can be a hard, bitter pill to swallow. And it is okay to get angry, just don't let your emotion of anger get the best of you and lead you into sin! Resolve your issues of anger, do not dwell on them, do not ignore them, do not let them fester. To dwell on it, to live in it, to not resolve it is to give the devil a foothold in your life.
I don't think anyone who truly walks in Christ wants that, do they?