To the lady chosen by God and to her children, whom I love in the truth—and not I only, but also all who know the truth— (2 John 1:1)
The writer of this short letter is traditionally considered to be the Apostle John toward the end of his long life. But in this letter, he identifies himself simply as the elder. This was likely a title of respect given to him by those he ministered among.
This letter is addressed to the lady chosen by God and to her children. And it ends with greetings from the children of her chosen sister. This lady and her sister would have been local church bodies. And their children would have been members of those churches.
The New Testament speaks of the Church as the bride of Christ (Eph. 5:28-32; Rev. 19:7; 21:2, 9; 22:17). And it would seem that John has a similar idea here. This lady, together with her sisters scattered around the world, make up the Church, the bride of Christ. And we, as members of the Church, are her children. God is my heavenly Father, and I have been born of the Spirit (John 3:5-8). But the Church, as it carries out the commission Christ left for her (Matt. 28:18-20), is instrumental in our new birth. We are her children.