Being "in" someone represents having a close relationship with him, that is being united to him or belonging to him. The phrase "him who is true" refers to the true God, and the phrase "in his Son Jesus Christ" explains how we are in him who is true. AT: "we are united to him who is true by being united to his Son Jesus Christ"
He is called "eternal life" becasue he gives us eternal life. AT: "and the one who gives eternal life" (See: [[rc://en/ta/man/translate/figs-metonymy]])
John was an elderly man and their leader. He used this expression to show his love for them. See how you translated this in [1 John 2:1](../02/01.md). AT: "My dear children in Christ" or "You who are as dear to me as my own children" (See: [[rc://en/ta/man/translate/figs-metaphor]])