"If you see your brother or sister committing what is not a mortal sin, you will ask, and God will give life to such a one—to those whose sin is not mortal. There is sin that is mortal; I do not say that you should pray about that. All wrongdoing is sin; but there is sin that is not mortal" (1 John 5:16-17).
The author of the First Letter of John was writing about the year 100 C.E., at a time when there was great dissent and division among followers of Jesus as to what his ministry had meant and how it had been expressed. This letter seems especially concerned with some who believed that Jesus had not been fully human. It expresses confidence that the congregation has the faith and strength to separate the true from the false. It also suggests as a kind of litmus test that beliefs about Jesus must be rooted in, and expressive of, the same divine love that he taught and demonstrated.
In addition to—and maybe more than—his concerns about false teachings, the author of the letter seems concerned that the spiritual community will give way to suspicion, mistrust and judgments as people accuse each other of not believing correctly. In this epilogue to the letter (5:14-21) he seeks to calm fears by assuring that minor infractions can be easily dissolved by the power of collective prayer.
The author isn't specific or clear about just what a 'mortal' sin might be. Clearly it's something that cannot be dissolved by communal prayer alone. In context—and using the teachings of Jesus as a guide—I suspect the more grievous sins are those that lead others away from their spiritual Truth. Metaphysically we understand 'sin' to be error thought. The word itself is rooted in the Greek for 'missing the mark,' as an archer might miss a target he fully intended to hit. This type of 'sinner' does not intend to go wrong; it is his false belief about how to hit the target that causes the arrow to stray. This type of 'sin' can be dissolved by 'repenting'—re-evaluating the situation, learning from the miss and aiming more accurately the next time. The prayer support of a spiritual community can be extremely helpful in supporting us through this process.
Suppose, however, that instead of correcting the 'error thought' that led to the sin, the individual continues to insist on doing things his way—and on persuading others to accept his misguided choice rather than trusting their own divine guidance. Such a person would be doing grievous harm to the innocent. Such entrenched, fear-based insistence on maintaining error thought cannot often be dissolved—'forgiven'—by human effort, however earnest and well-intentiioned. It requires a realignment with the Divine.
So the letter ultimately endorses Jesus' repeated teaching that we refrain from judging each other. Minor expressions of error thought do not need judgment; they can be corrected through collective prayer. And major expressions are not ours to judge. They must be left to God.