en_tn_lite_do_not_use/1jn/03/10.md

In this the children of God and children of the devil are revealed

This can be stated in active form. Alternate translation: "This is how we know who the children of God are and who the children of the devil are" (See: rc://en/ta/man/translate/figs-activepassive)

Whoever does not do what is righteous is not from God, neither is the one who does not love his brother

"whoever does not do what is righteous is not from God; whoever does not love his brother is also not from God" or "those who are from God do what is right, and those who are from God love their brothers"

his brother

Here "brother" means fellow Christians.