The fact that Old Testament law is no longer applicable post-crucifixion is indisputable. The New Testament repeatedly affirms that the cross ushered in a new covenant between man and God; one in which the law that couldn't save us is replaced by a law that can (Romans 8:3). The law brings the knowledge that salvation is necessary, and points us to the One who can save us, but it does not save us. Now that Christ has come, Paul explicitly tells us that we're no longer under it (Galatians 3:24-25).
Despite being such an indisputable fact of scripture, many people do attempt to dispute it. Whether an atheist seeking to prove Christianity is necessarily violent, or an over-zealous Legalistic Christian, there are enough people who believe Old Testament law still applies today that if you preach the Gospel to a wide enough audience, this will come up. One verse in particular appears very frequently.
In Matthew 5:17-18, Jesus says "Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled." Legalists and unbelieving apologists use this verse to show that Jesus did not come to abolish the law, because not one jot or tittle can be removed from the law until all is fulfilled. They are absolutely correct! Jesus did not come to abolish the Law or the Prophets. Neither do (most of) those who preach that we are no longer under the law believe He did. Right this second, I am also going to tell you that I don't believe the Law or Prophets have been abolished.
But there's more to that verse that literally every Legalist ignores: "I did not come to destroy but to fulfill." (Emphasis mine). So, first of all, Jesus draws a distinction between abolition/destruction and fulfilment. And well He should! But He not only makes that distinction, but in doing so, He also says He has come to fulfil it. Then He says the part about the law not passing away until all has been fulfilled. In other words, "hey, guys, I've come to do what is necessary to make the law pass away, because if I don't do it, it won't pass away".
Anyone who uses Matthew 15:17-18 to prove that the law is still in force must necessarily conclude that Jesus failed in His mission to fulfil it. But I can tell you with the greatest joy in my heart, He did not. As Christians, we are no more obligated to fulfil the law given in the Torah than we are to remain vegetarian (Genesis 1:29-30). Just as you can eat a hamburger despite the law given to Adam, so also can you eat a bacon butty despite the law given to Moses. When Jesus died, so did the law. When Jesus rose, the law didn't rise with Him. We are now under a brand new, far more glorious, far less hopeless, and in many ways far stricter covenant. A covenant so powerful that it allows a sinner like me to have a restored relationship with my Creator through the blood of His only Son. If you want to believe we are still under the law, you will find no scriptural support for your views.