While on the main Keller's right-on here, I think he himself muddles the important logical element of covenant theology which clarifies the consistency of Scripture even more.
The nation of (theocratic/covenantal) Israel is kaput--since 587 BC no less, hence the civil penalties of the law do not, and never have, been applicable to Christians--or anyone under the New Covenant. With the re-establishment of the monarchy in the Messiah Jesus, the New Covenant completely takes over, and the OT moral law is properly understood as the right response of an already saved people loving God, and loving neighbor.
Following OT law--NOT civil (as Israel is kaput) and NOT ceremonial (since Jesus fulfilled all those sacrificial/blood obligations and, God is making one people, no longer segregated by diet/custom/ethic boundaries)--rather MORAL (since God is still holy, and created morality for our good) is NOT based on the simplistic idea that if it is repeated in the NT it applies to us (contra Keller). That's a general principle that usually works...but a more correct approach is that OT moral teachings do now, and have always applied...but the Gospel puts the Law in its proper role of not saving us, rather guiding our Holy Spirit empowered love, once we are born again.
There are several specific OT moral prohibitions (bestiality, and incest for example) which are not actually specifically repeated in the NT, rather are assumed as wrong. (Yes I know legal incest was the issue in 1 Corinthians, but even then it is approached obtusely--with Paul expressing horror that they didn't grasp the obvious evil of it).
This is why it is important to know the whole bible--as the Old Covenant (OT) laws DO actually apply to us--in their moral codes, and should not be disdained as "oh, that's in the OT" as many ignorant (even evangelical) Christians approach them today. The NT ASSUMES a thorough knowledge of, and acceptance of the normative authority of....the OT. A sloppy approach of "only if it's in the NT does it apply" approach leads to compromising and crude ethics.
On a relatively minor issue for example, acceptance of the normative authority of OT law would avoid confusion amidst Christians today: Tattoos.
Rejected by the Church for 2000 years on the basis of one tiny OT (only) proof text: "Do not cut your bodies for the dead or put tattoo marks on yourselves. I am the LORD." (Lev 19:28), none the less many Christians enthusiastically embrace the tattoo trend today. Interestingly "I am the LORD." is the reasoning in Leviticus..ie, God owns (even) our bodies.
Paul reiterates and elaborates the principle in 1 Cor. 6:19,20: "Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your bodies."
Paul in context is calling for sexual purity. However purity for the body isn't limited to sexual activity.
Without an acceptance of the normative authority of OT ethical/moral commands, and therefore an ignorance of the OT--even over something as minor as tattoos--evangelicals today skate right over NT commands elaborating on the principle of God's ownership of our bodies, and do graffiti on the temples of God.
