In summary, it all comes down to Jesus.
"I don't expect everyone to understand that the whole Bible is about Jesus and God's plan to redeem his people, but I vainly hope that one day someone will access their common sense (or at least talk to an informed theological adviser) before leveling the charge of inconsistency," the New York pastor wrote.
Before Jesus' time, God's people – as recounted in the Old Testament – had to offer sacrifices to atone for sin and follow a complex set of rules for ceremonial purity and cleanness. This included eating certain kinds of foods while abstaining from others (shellfish), and wearing certain forms of dress (garments woven with two kinds of material). That was the only way one could approach God in worship, Keller explained.
Today, such rules are not followed by Christians because of what Jesus Christ did on the cross, the pastor said.
"When he died on the cross the veil in the temple tore, showing that he had done away with the need for the entire sacrificial system with all its cleanliness laws. Jesus is the ultimate sacrifice for sin, and now Jesus makes us clean," he stated.
And it's not that Jesus abolished those Old Testament laws; rather, he fulfilled them.
"Whenever we pray 'in Jesus name' we 'have confidence to enter the Most Holy Place by the blood of Jesus' (Heb. 10:19)," Keller added.
Therefore, he clarified, to continue following the ceremonial laws would actually be inconsistent with the teaching of the Bible.
So when charged with inconsistency, the Christian should respond, "If I believe Jesus is the resurrected Son of God, I can't follow all the 'clean laws' of diet and practice, and I can't offer animal sacrifices. All that would be to deny the power of Christ's death on the cross. And so those who really believe in Christ must follow some Old Testament texts and not others," said Keller.
There are still laws that are binding to this day, he noted.
"In short, the coming of Christ changed how we worship, but not how we live."
Which laws are still in force?
The New Testament reaffirms laws that are still to be obeyed. They include loving one's neighbor, caring for the poor, and not committing adultery or killing.
Keller also pointed out, "[A]ll the sex ethic of the Old Testament is re-stated throughout the New Testament."
Some of the arguments he hears often is that Christians "ignore" certain Old Testament texts, such as the rule about not eating shellfish or raw meat, and then they condemn homosexuality.
"It's not only the Old Testament that has proscriptions about homosexuality," he argued. "The New Testament has plenty to say about it as well. Even Jesus says, in his discussion of divorce in Matthew 19:3-12, that the original design of God was for one man and one woman to be united as one flesh, and failing that (v. 12), persons should abstain from marriage and sex."
The penalty for sins has changed, however. Whereas in the Old Testament, sins like adultery or incest were punishable with civil sanctions like execution, such punishment does not apply to God's people today.
Why? Because the people of God no longer constitute a nation-state. They are now an assembly of churches who live under many different governments around the world.
"The church is not a civil government, and so sins are dealt with by exhortation and, at worst, exclusion from membership," said Keller.
Ultimately, it comes down to Christ. "If Christ is God, then this way of reading the Bible makes sense," he said. But if one rejects that main premise of Jesus as the resurrected Son of God, then the Bible is "at best a mishmash containing some inspiration and wisdom, but most of it would have to be rejected as foolish or erroneous."
"But you can't say in fairness that Christians are being inconsistent with their beliefs to follow the moral statements in the Old Testament while not practicing the other ones," Keller argued.