There are portions of the Bible that are difficult to understand. Peter says so (2 Pet. 3:16). However, the Bible also contains many instructions that are amazingly simple.
In Ephesians 4:25-32, Paul describes the change in conduct that takes place when one puts off the old man and puts on the new man (vs. 22-23). We will stop practices like lying, living in anger, stealing, and cussing. We will put away evil attitudes like bitterness, wrath, anger, and malice. These old things must be replaced with new things. The first new thing mentioned by Paul is to "be kind to one another" (v. 32).
Being kind to another person requires me to be good, gentle, and helpful. Instead of ignoring the problems of others (like the priest and Levite), kindness requires me to be like the Samaritan and help my neighbor (Luke 10:30-37).
This simple instruction is very powerful and far reaching. Petty little issues cannot grow into major problems when we are kind to one another. When we treat one another the way we want to be treated, give one another the benefit of the doubt, encourage one another to do and be our best, weep with those who weep and rejoice with those who rejoice, etc., we are being kind to one another. Doing this will allow us, as a congregation, to devote our time and attention to the Lord's work.
"Be kind" is a present imperative, which means the action is to be continued. It is not a one-and-done responsibility, but a way of life. We are expected to learn and live the basics. Moving on to spiritual maturity does not mean we forget the first principles. The High School graduate is expected to remember the things he learned in Kindergarten. There is no excuse for us not being kind to one another.