"The church has to be everywhere there are people but the people are moving into the city faster than the church. If you love what God loves, you will love the city.
"If you want to go where the people are going you've got to go to the city. And our churches are not going to the city nearly as fast as the people are."
Just 40 years ago, less than 3 per cent of the world's population lived in cities. Today, an estimated eight million people make the move to a city every two months.
If churches want to reach young adults, business leaders, cultural influencers and the poor, then they have to be in the cities because that is where so many of them are living, he said.
"The most unreached peoples in the world go to the cities," he explained. "When they immigrate to the cities they break their kinship ties, they are in a more pluralistic environment, and are far more open to the Gospel than they ever would have been in their previous habitat."
Keller is the founder and pastor of Redeemer Presbyterian Church, a large and growing congregation in Manhattan. He admitted that running a city centre church could entail several "headaches" for pastors.
He said that the level of diversity meant they had to be extremely patient with charges of cultural insensitivity and deal with conflict in a way that rural churches did not have to.
With work taking up a much bigger part of people's lives in the city than in rural areas, churches have to show people how to relate their faith to the work rather than expect to take them out of their work world into the church world.
Most importantly, he said urban churches had to cooperate with other churches, denominations and traditions in a way they could afford not to elsewhere.
"In the city you will never reach the city unless you are very cooperative with believers across denominational lines.
"All of those things outside the cities are optional, it would be nice if you had them in your church. But in cities they are absolutely necessary," he said.
He said many Christians were "defeatist" about evangelising cities but he told Christians it could be done by praying for them and showing "neighbour love".
"We should sacrificially lay our lives out for the people in this city. People should see that we care about them, that we love them.
"Most of all we should offer the righteousness of Jesus Christ to cover their sin, that they may be saved, because he is the priest that those cities need. We, believing in him, can be the priests that our cities need.
"Look at the cities of the world, look at the masses. God is saying 'why aren't you moved by them?' 'Why aren't you going there?'
"So let's go."