Here John probably means that believers' ability to love God and each other sincerely ([1 John 3:18](../03/18.md)) is a sign that their new life has indeed originated from the truth about Christ.
People living according to the truth is spoken of as they were "from the truth." Alternate translation: "we are living according the way Jesus taught us" or "we belong to him who is true" (See: [[rc://en/ta/man/translate/figs-metaphor]])