When we go out and share the knowledge of God's great love we reflect something very profound about who God is. For when Jesus sends us, he is allowing us to share the missional, generous, outgoing shape of God's own life. The writer of Hebrews puts it like this: "Jesus also suffered outside the city gate [that is, he went out beyond where the people of God are] to make the people holy through his own blood. Let us, then, go to him outside the camp (Hebrews 13:12-13). In other words, Jesus is found out there, in the place of rejection. That is where the Father has sent him, that he might bring sinners back as children. The Christian life is one of being where he is, of joining in how he has been sent.
And the motivation? Well, why did the Father send the Son? Because the Father so enjoyed loving the Son that he wanted his love to be in others. John 17:25-26 says:
"Righteous Father, though the world does not know you, I know you, and they know that you have sent me. I have made you known to them, and will continue to make you known in order that the love you have for me may be in them and that I myself may be in them."
And why did the Son go? Because, he says, "I love the Father and... I do exactly what my Father has commanded me" (John 14:31). So the Father sent the Son because of how he so loved him (and wanted the love to be shared and enjoyed), and the Son went because he so loved his Father (and wanted that love to be shared and enjoyed). The mission comes from the overflow of love, from the uncontainable enjoyment of the fellowship.
So it is with the Father and the Son; so it is with us. The Spirit catches us up to share their pleasure, and it is that delight in them that fuels us to want to make them known. The Spirit-caused enjoyment of the fellowship, the increasing love for the Father and the Son: it turns us to share their outgoing love for the world. We become like what we worship.
The Puritan Richard Sibbes once said that a Christian singing God's praises to the world is like a bird singing. Birds sing loudest, he said, when the sun rises and warms them; and so it is with Christians: when they are warmed by the Light of the world, by the love of God in Christ, that is when they sing loudest.
As the shining of the sun enlarge th the spirit of the poor creatures, the birds, in the spring time, to sing, so proportionably the apprehension of the sweet love of God in Christ enlarge th the spirit of a man, and makes him full of joy and thanksgiving. He breaks forth into joy, so that his whole life is matter of joy and thanksgiving.
Sibbes was quite right, for "out of the overflow of the heart the mouth speaks" (Matthew 12:34). If I don't enjoy Christ, I won't speak of him. Or, perhaps worse, I will, but without love and enjoyment- and iffy mouth does give away my heart, people will hear of an unwanted Christ. And who would want that?
The Spirit, of course, can use such loveless evangelism. But his real work is to bring us to, and keep us in, the sunshine of God's love. It is there that we will sing heartily; it is there, abiding in Christ, that we will bear fruit. The Spirit shares the triune life of God by bringing God's children into mutual delight of the Father and the Son- and there we become like our God: fruitful and life-giving.