God, in his providence, designed all the events of the first century Church so that various crises came up in churches when apostles couldn't get there soon enough, or in person. So they wrote letters.
When courtesy becomes cruelty.
God is our shield and our great reward.
In his epistle, John exhorts us to follow God's commandment of loving one another.
The light of the kingdom of God is shining in this world, and we are instruments of adavancing that kingdom through the gospel.