It is easy to miss great spiritual truths when we fail to look at and understand the context in which they are given. This parabolic illustration by Jesus Christ is not about trees and the context makes this clear. In the context of the Sermon on the Mount and chapter 7 in particular, Jesus gives us a a life principle that is very important in our relationships with other believers. We are not to judge one another but we are to be discerning. What some call the Golden Rule in verse 12 is a guiding principle by itself. We are to be discerning whether or not a tree (brother or sister in Christ) is yielding good fruit or evil fruit. You may discern that some are not truly Christians and these must be avoided lest you are influenced by their evil fruits.