The Archbishop of Canterbury said he had "no fears" for the future of the Church in Britain.
Urging all Christians not to wait before going out to share their faith, he said evangelisation was not a survival strategy for the church, but an activity "central to being the people of God".
He was speaking days after one of his bishops said the Church of England has "five or six years" to save itself.
Bishop of Truro Tim Thornton told BBC Radio Cornwall that radical change was needed to halt a steady decline. Bishop Thornton had said the Church of England will struggle to exist in 10 years, but has shortened his estimate of its survival span to nearly half that. "I'd been saying that for a while now, so I think we have to come down and say it's five or six years."
Analysis of attendance figures was "all showing one thing". The Bishop said: "I fear that we are on a steady decline at the moment."
Archbishop Welby, who has made evangelisation a priority of his ministry at Lambeth, told the Church Army's annual meeting in London: "We can't say we're going to use God to ensure the church keeps going. The God we worship is a God who sends out and harvests in. And we see that most clearly in the person of Jesus Christ. Jesus is God himself, sent out to sow, to gather, and to draw back in.
"So what you're doing in evangelism isn't a tool of the church, it's not some sort of thing we do so we can still be there in a generation or two. It is the revelation of the nature of God. It is intrinsic to being the people of God."
The Archbishop said he had "no fears about the future of the church of God" in this country.
"But I would really like, and I believe it's part of God's plan, that that future of the church of God includes a renewed, invigorated, confident, wonderful, celebratory, humble, delighting Church of England."
He said that sharing Jesus with others is not something that churches should wait until they are "ready" before doing.
"Worship and evangelism... neither are things you wait to be ready to do. You do them. You get on with them. That is the nature of the God we serve."
He said evangelism must be grounded in theology.
"To earth our activity in theology, in who God is, is incredibly important. Because when we understand that this is the God we serve, that this is intrinsic to his nature, who he has revealed himself to be, that tells us that what we're doing is not just one of those things that some people do and some people don't – this is absolutely fundamental to being the people of God.
"And that's quite encouraging, because when we're being the people of God, the one thing we can be pretty sure about is that God will show up."