And if Christians question whether the poor person will abuse their charity and grace, then they should remember that they have also "trampled" on God's charity and grace by not living the life they should.
"The more you see yourself as spiritually poor and the recipient of God's wonderful grace the more your heart is going to go out to the poor," said Keller, founding pastor of Redeemer Presbyterian Church in New York City. "When you look at the materially poor you know that in a sense you are looking in a mirror."
Keller, a New York Times bestselling author, was discussing the theological ideas behind his latest book, Generous Justice: How God's Grace Makes Us Just. In the book, Keller calls Christians, particularly evangelicals, to be involved in social justice and to understand deeply the biblical basis for such activities.
The influential pastor pointed to Deuteronomy 15:4 ("However, there need be no poor people among you, for in the land the LORD your God is giving you to possess as your inheritance, he will richly bless you") and Acts 4:34-35 ("that there were no needy persons among them. For from time to time those who owned land or houses sold them, brought the money from the sales and put it at the apostles' feet, and it was distributed to anyone who had need") to show that there is continuation from the Old to the New Testament about the idea that God's people should be generous by canceling debts and sharing their possessions so that everyone has enough.
Another biblical point about charity is when Paul gathered resources from churches with more money to help poor Christians who were suffering from a devastating famine. Paul's action supports the idea that those with more should share with the poor so that there are no poor and needy people among them.
But being generous will cost the giver something, Keller stressed. Being generous can require carrying another person's burden. To bear someone's burden means that a certain amount of the burden falls on the giver.
"The more you understand the gospel of grace with the mind and experience with the heart, the more likely you are to care about people who are poor, marginalized, [and] hungry," said Keller. "I think that is a very important biblical theme."
The pastor, known for his deep thinking, said he was motivated to write the book to address the concerns of Christians on the issue of social justice. One Christian camp is overly zealous on the issue while the other side is afraid that taking part in social justice will "smother the primacy of the ministry of the word in the life of the church."
But Keller sees justice as empowered by an experience of grace. The NYC pastor defines justice as simply giving everyone what they deserve according to what God thinks is fair to give to those made in His image.
"Justice is not part of the gospel, but anyone who understands the gospel it moves you towards living a just life," he said.
Keller's latest book also guides Christians on whether they should work with non-Christians on social justice issues and discusses how to have a more just society.
Redeemer Presbyterian Church was founded in 1989 and has more than 5,000 people that attend it regularly on Sundays.