I rejoiced greatly that I found - That I learned this fact respecting some of thy children. The apostle does not say how he had learned this. It may have been that he had become personally acquainted with them when they were away from their home, or that he had learned it from others. The word used εὕρηκα heurēka would apply to either method. Grotius supposed that some of the sons had come to Ephesus on business, and that John had become acquainted with them there.
Of thy children walking in truth - That is, true Christians; living in accordance with the truth, for this constitutes the essence of religion. The expression used here, "of thy children," (ἐκ τῶν τέκνων ek tōn teknōn,) means some of thy children; implying that he knew of a part of them who were true Christians. This is clear from the Greek construction, because:
(a) if he had meant to say that he had found them all to be of this description, the sentiment would have been directly expressed, "thy children;" but as it is, some word is necessary to be understood to complete the sense; and,
(b) the same thing is demanded by the fact that the participle used ("walking" - περιπατοῦντας peripatountas) is in the accusative case.
If he had referred to them all, the participle would have been in the genitive, agreeing with the word "children," (τῶν περιπατοῦντων tōn peripatountōn) - Lucke. Whether the apostle means to say that only a part of them had in fact embraced the gospel, or that he had only known that a part of them had done it, though the others might have done it without his knowledge, is not quite clear, though the former supposition appears to be the correct one, for if they had all become Christians it is to be presumed that he would have been informed of it. The probability seems to be that a part of her children only were truly pious, though there is no evidence that the others were otherwise than correct in their moral conduct. If there had been improper conduct in any of her other children, John was too courteous, and too delicate in his feelings, to allude to so disagreeable a circumstance. But "if that pious lady," to use the language of Benson, "had some wicked children, her lot was not unique. Her consolation was that she had some who were truly good. John commended those who were good, in order to excite them in the most agreeable manner to persevere."
As we have received a commandment from the Father - That is, as he has commanded us to live; in accordance with the truth which he has revealed. The "Father," in the Scripture, is everywhere represented as the Source of law.

That I found of thy children walking in truth - I have already supposed this Christian matron to be mother of a family, probably a widow, for no mention is made of her husband; and that she was also a deaconess in the church, and one in those house the traveling evangelists preached, and there they were entertained. The children mentioned here may either be her own children, or those members of the Church which were under her care, or some of both. The apostle was glad to find, probably by an epistle sent from herself to him, or from the information of some of the itinerant evangelists, that the work of God was prospering in the place where she lived, and also in her own household. He does not say that all were walking in the truth, but εκ των τεκνων, some of her children; there was a growing and spreading work, and there were many adversaries who strove to pervert them who had already believed, and perhaps were successful in drawing several away from their simplicity.

(2) I rejoiced greatly that I found of thy children walking in (d) truth, as we have received a commandment from the Father.
(2) This true profession consists both in love towards one another which the Lord has commanded, and also especially in wholesome and sound doctrine, which also is delivered to us: for the commandment of God is a sound and sure foundation both of the rule of conduct and of doctrine, and these cannot be separated from one another,
(d) According as the truth directs them.

I rejoiced greatly that I found of thy children,.... Not all, but some of them; for good parents have not always good children, or at least not all of them; Adam had a Cain, Abraham an Ishmael, and Isaac an Esau: God is pleased to show his discriminating grace in tribes and families, by taking some, and leaving others: it is a great mercy when any are called by grace, and instead of the fathers are the children: and this was the case of some of the children of this elect lady, they were
walking in the truth: in Christ, the truth, by faith, as they had received him; and in the truth of the Gospel, as they had embraced and professed it; they abode in it, and by it, and made a proficiency in the knowledge of it, which may be signified by walking, that being a progressive action: as also they walked according to it, and as became it; and likewise they walked in the truth of Gospel worship, discipline, and ordinances:
as we have received a commandment from the Father; and which has been made known by Jesus Christ, as his mind and will. Now as it is matter of joy and gladness to godly parents when their children walk in the paths of faith, truth, and holiness, so it is also to ministers of the Gospel, as well as to the angels in heaven; it gives them an inward pleasure and joy, and which is not only expressed by them, to such children and their parents, but is also abundant by many thanksgivings unto God.

It is good to be trained to early religion; and children may be beloved for their parents' sake. It gave great joy to the apostle to see children treading in their parents' steps, and likely in their turn to support the gospel. May God bless such families more and more, and raise up many to copy their example. How pleasing the contrast to numbers who spread irreligion, infidelity, and vice, among their children! Our walk is true, our converse right, when according to the word of God. This commandment of mutual Christian love, may be said to be a new one, in respect of its being declared by the Lord Christ; yet, as to the matter, it is old. And this is love to our own souls, that we obey the Divine commands. The foresight of the decay of this love, as well as of other apostacies, or fallings away, might engage the apostle to urge this duty, and this command, frequently and earnestly.

I found--probably in one of his missionary tours of superintendence. See Introduction, at the end, and 2-John 1:12; 3-John 1:10, 3-John 1:14.
of thy children--some.
in truth--that is, in the Gospel truth.
as--even as. "The Father's commandment" is the standard of "the truth."

I rejoiced greatly. Because he had found her children walking in the truth, by keeping the commandment of the Father. The commandment referred to, the one which sums up all, is love.
And now I beseech thee, lady. Kyria in Greek. If lady at all it would mean "your ladyship."
Not as though I wrote a new commandment. Love embraces all. Compare 1-John 2:7.
And this is love, etc. Keeping the commandments is the demonstration of love. Compare 1-John 5:3, and John 14:15, John 14:23.

I found of thy children - Probably in their aunt's house, 2-John 1:13. Walking in the truth - In faith and love.