Here is what the Glossa Ordinaria says
He that knows his brother. He is talking of the everyday and light sins which are healed as easily as they are difficultly avoided. If you have sinned in word, in thought, by forgetfulness or by ignorance, confess to a brother, as James teaches, ask him to intercede for you (Jm. 5). And if he confesses to you in his turn, intercede for him too. Here the prayer of the Lord suffices, and mutual confession. It is a light penitence. Then if the sin is a serious one, bring in the priests of the church, and chastise yourself after their examination. (Bede Expo. On 1 John)
There is a sin to death which separates you from God as death separates the soul from God. There is a sin to death.
The Lord orders to pray for persecutors; here he (John) says that there are some people for whom you must not pray, because there are sins in brothers that are more serious than the persecution of enemies; the sin of a brother is to death when, after the knowledge of God was given to him through grace of God, someone attacks the brotherhood, and is agitated by the firebrands of envy against the grace by which he was reconciled. It is a sin not to death if someone has not removed his love from his brother, but through some weakness of the mind has not fulfilled the duties of brotherhood, whence Christ, “Father, forgive them, for they know not what they do” (Lk. 23:34), for they had not yet been made sharers through the grace of the Spirit and had not yet entered the society of the holy brotherhood. So did Stephen too pray for those who had not received God’s grace. Paul does not pray for Alexander; because, as he was already a brother, he was attacking the brotherhood. For those who had yielded out of fear he does pray. All left me alone but may it not be laid to their charge (ref. 2 Tim. 4:16). (Bede Expo. On 1 John)
Because that which is not corrected in this life, its pardon is hoped in vain after death; it can also be taken as referring to any criminal sin. Whatever differs from the measure of equity is numbered among sins, but the very little sins are not prejudicial to the just; but some sins are so opposed to all justice that if they are not corrected they plunge you into punishment.