en_tn/1jn/02/05.md

keeps his word

To keep someone's word here is an idiom for to obey. AT: "does what God tells him to do" (See: :en:ta:vol1:translate:figs-idiom])

in him truly the love of God has been perfected

This can be stated in active form. Possible meanings are 1) "the love of God" refers to a person loving God, and "perfected" represents completely or fully. AT: "it is that person who loves God completely" or 2) "the love of God" refers to God loving people, and "perfected" represents completing its purpose. AT: "God's love has achieved its purpose in that person's life" (See: rc://en/ta/man/translate/figs-possession and rc://en/ta/man/translate/figs-activepassive)

By this we know that we are in him

The phrase "we are in him" means that the believer has fellowship with God. AT: "When we obey what God says, we can be certain that we have fellowship with him" or "By this we know that are joined to God" (See: rc://en/ta/man/translate/figs-metaphor)