"As you, Father, are in me, and I am in you, may they also be in us, so that the world may believe that you have sent me."
Jesus had taught his disciples the importance of "abiding" in him (see "vine & branches," ch.15!). Jesus modeled that, too... in his relationship with God. Now that he's near the completion of his ministry on earth, he's praying for his followers. He prays for abiding intimacy. That we, as his disciples, may abide in him and in God the Father. Why? For our own fruitfulness, of course... but something more. Jesus wants us to abide in him/God so OTHERS may believe that he is who he is!
As I read & reflect on this, I don't see it as a command to preach. Or to convert others. Or to argue theological "truths." I see this as a call to live out of that "abiding" connection. Live life like Jesus did. Let others see the authenticity of our lives... and let that point them to THE ONE who came from God: Jesus.
We abide when we spend time with God. Scripture reading/journaling, prayer, worship, study, relationships/small groups, etc. I need to keep growing in my "abiding," so others can come to know Jesus. It's that important.