This is a strong statement from our Savior! The fundamental question is: "How Christ-like is our speech?" This is just one area out of the whole of our behavior. We will be judged for every word, even the idle ones that we may just toss off in a time of weakness or when joking around with friends. That is a pretty strict judgment.
Jesus speaks here in black-and-white terms. The tree (meaning the person) is either good—producing good fruit—or he is bad and produces bad fruit. Which are we—the good or the bad tree?
In verse 34, He says, "Out of the abundance of the heart we speak"—and we could add, "and act." Jesus says in Matthew 15:17-18 that it is not what goes into a man that defiles him, but what comes out of him. What comes out of a person will be either good things like service, love, kindness, and other fruits of the spirit, or bad things, the works of the flesh, which He names there.
So, what will it be with us? What is the abundance of our heart?
The picture here is that the heart is a kind of vessel—a bowl—and things are poured into the heart. At a certain point, the vessel will overflow, and an abundance will come out of it. What comes out of our heart—this bowl or vessel—will expose the characteristics of the heart.
When we pour information into our minds, we process it. For a while, it stays in the bowl, as it were, and becomes mixed with what has been put there before. Our minds work on it for a while, and over time, it begins to gel into certain ideas. Once our minds are full, ideas break out in words, plans, and behaviors. Evil thoughts within, evil speech and/or works without. Or, we can put it the other way around—godly, kind, Christ-like thoughts within, godly, kind, Christ-like speech and/or works without.
What breaks out of our hearts? We have to answer that ourselves. Do we have profane minds that spew out profane speech? Or, is it "on [our] tongue is the law of kindness" (Proverbs 31:26) because behind our tongues are pure and kind hearts?
This is vitally important because "by those words" we will either be justified or condemned. Our thoughts are just precursors to our speech and action.
So, where do we stand in relation to this line that Jesus Christ our Savior, our High Priest and Judge, has drawn? Are we a good tree or a bad one?