This is how we will know that we belong to the truth, and will set our hearts at rest in his presence.
In context
17If anyone has worldly goods and sees a brother or sister in need, but closes his heart against him, how can the love of God remain in him? 18Dear children, let us not love with word or speech, but with action and truth. 19This is how we will know that we belong to the truth, and will set our hearts at rest in his presence. 20For whenever our hearts condemn us, God is greater than our hearts, and he knows everything. 21Beloved, if our hearts do not condemn us, we have confidence before God,