John Gill emphasizes that true believers demonstrate their identity as children of God through their love for one another, which serves as evidence of their faith and connection to the truth of God. This love is not merely verbal but is expressed in actions, reflecting the nature of Christ, who embodies truth and love. Gill explains that such love assures believers of their standing before God, providing them with peace and confidence in their relationship with Him. He contrasts this with the turmoil experienced by those who harbor hatred, underscoring the importance of brotherly love as a hallmark of genuine faith. Ultimately, believers can approach God with boldness, knowing their love for others is recognized by Him.

Ver. 19. And hereby we know that we are of the truth,.... By the saints loving one another in deed and in truth, they know, as the cause is known by the effect, that they are of God, who is the true God, the God of truth, and cannot lie, and is truth itself; that they are the children of God, and are born of him, since they love those that are, and every like loves its like; and that they are of Jesus Christ, who is the way, the truth, and the life; that they belong to him, are his, since they have his Spirit, as appears by his fruits in them, and this, among the rest, love to the brethren; and that they are his disciples, which others, even all men know, as well as themselves, by their mutual brotherly love; and that they are of the Gospel, which is truth, and the word of truth; that they are begotten, and born again, according to the will and grace of God by it, and are on the side of it, and can do nothing against, but all for it; and that they are true, sincere, and upright persons, true believers in Christ, whose faith works by love, and are real lovers of him, and his, since they love not in word only, but in deed and in truth. And shall assure our hearts before him; or "persuade our hearts": arrive to a full assurance of faith, hope, and understanding, that we are of the truth, do belong to God, are loved by him with an everlasting love, are chosen by him unto salvation, and are his adopted and regenerated ones, having passed from death to life, of which brotherly love is a sure evidence, 1Jo 3:14. Some render the words "shall pacify", or "make our hearts tranquil": or "quiet"; this only the blood of Christ can do, and does, being sprinkled on the conscience: he only has a quiet mind, or true peace of conscience, that looks to the righteousness of Christ for justification, and deals with his blood for the full and free remission of his sins: it is true indeed, that one that loves his brother heartily and sincerely, has peace of mind in it, though not for it; when, on the other hand, there is no peace to the wicked man, that hates his brother; for where there is envying, malice, hatred, and strife, there is no true peace, pleasure, and comfort, but confusion, uneasiness, distraction, and every evil work. Or this passage may refer to that holy confidence before God, which true believers in Christ, and cordial lovers of the brethren, have; both now at the throne of grace, where they can come with boldness, intrepidity, and freedom, to ask for what they want, and confidently believe they shall receive what is proper and needful for them; and also hereafter, at the throne of judgment, and in the day of judgment, when they shall have boldness, and not be ashamed before the Judge at his coming; who will particularly take notice of their love in feeding, clothing, and visiting the least of his brethren, which he takes as done to himself.