1 John 1:10

If we say that we have not sinned, we make Him a liar, and His Word is not in us.

What St. John here discusses is the heresy of perfectionism, the idea which is held by many people to this day, namely, that they can attain to such a perfect state in this world that they are completely free from sin in their own persons, "in an uninterrupted obedience. " To these the apostle says: If we say that sin we do not have, ourselves we deceive, and the truth, not is it in us. The very position of the words expresses the horror which John must have felt at the mere suggestion of such blasphemy. There is no such thing as perfect sanctification in our own persons in this life, making the forgiveness of sins superfluous so far as we are concerned. If anyone should hold this foolish notion and even confess it, he is deceiving himself, he is leading himself astray, he is leaving the eternal truth as revealed in the Word of God. He is denying the truth that all men have sinned and come short of the glory of God, that there is none that doeth good, no, not one. He has left the truth that we sinners are justified before God by grace, for Christ's sake, through faith. Thus the truth will no longer be in such a person, he is lost in the blindness of self-righteousness, he has lost the fellowship with God and with Jesus Christ, his Savior.

But, on the other hand: If we confess our sins, He is faithful and just, that He forgives the sins and cleanses us from all unrighteousness. That is the custom which the Christians have, to bring their transgressions before their heavenly Father in contrition and repentance, to confess them all without excuse or attempt at mitigation. We can do that so freely because we know that God is reconciled to us through the blood of His Son. He forgives us our sins for the sake of Christ, He cleanses us from all our imperfections and unrighteousnesses, from the sins which still cling to us and make us laggards in the fulfillment of God's will. This He can do because the righteousness of Christ is there in sufficient quantity to outweigh all our trespasses; His expiation is great enough to cover all our sins. What is more, in doing this, our heavenly Father is proving Himself faithful to His promises, Hebrews 10:23. And He is just; having accepted the redemption of Christ, His perfect reconciliation, it would be an act of unrighteousness and injustice on His part to break His promise ratified by the blood of Jesus. If Christ were still in the grave, then our hope were vain; but with the risen Christ exalted to the right hand of God we are courageous and defiant in faith.

The apostle again lifts his finger in warning to check the pride and self-righteousness of our hearts: If we say that we have not sinned, we make Him a liar, and His Word, not is it in us. If any person that is at all acquainted with the Word of God is so blind and perverse as to deny his own sinfulness, he is stifling the voice of his conscience, he is setting aside the entire Word of the Gospel, he is rejecting the entire experience of mankind. Thus he makes God a liar; for the entire content of His Word may be given in the two words, sin and grace; and he most assuredly has not the faintest conception of the truth of God as it is contained in His revealed Word. Let every Christian, therefore, guard against such a delusion with all vigilance, and to that end make the study of God's Word a daily practice. Then his own sin, but above all the greatness of God's mercy, will be revealed to him with ever greater emphasis.

Summary. The apostle gives a brief summary of the doctrine concerning the person and the office of Christ, showing at the same time that God is Light and that we should walk in this light, recognizing and acknowledging our sins, but also the forgiveness of God through the blood of Christ.