NOTES for Jo1 4:2-3
These words from the First Catholic Epistle of the apostle John are very important. The apostle tells us of the need to test the spirits, that is, to distinguish what is God's and what is satanic in life. Before these words the apostle emphasizes that faith, keeping the commandments, and a heart open before God are characteristic of a Christian. And he formulates the famous words sometimes called "the criterion of the apostle John." Every spirit, says the apostle, that confesses Christ come in the flesh is from God. If we look attentively at the history of the dogmatic development of Christianity in the age of the Ecumenical Councils, we will see that in the final analysis all false teachings were rejected by the Church precisely according to this criterion. And, conversely, conformity to it helps distinguish Christian confessions from quasi-Christian sects.
But besides their dogmatic and historical significance, these words are also very important in the practical life of Christians. First of all, the apostle John speaks about discerning the spirits that speak in a person's heart. What is right and what is wrong, what is worthy of a Christian and what is unworthy — to a significant degree, in order to discern this in one's own life, one needs to use precisely the criterion of the apostle John.