The argument of the atheists and secularists is invalid, but it is still effective. That’s because trying to explain why it is invalid requires quite a bit of exposition, a lot of which is only comprehensible to people already familiar with the Bible and Christian theological ideas. They can toss this grenade out, and by the time you could defuse it, the audience has lost interest and walked away with the impression that you are a hypocrite.
One way to counter that is simply to flip the accusation back at them. Tell them that Muslims are completely inconsistent when it comes to following verses from the Quran. Then demand to know why they are not out publicly condemning the Muslims for their inconsistency. If they tar you unfairly as a hypocrite, tar them right back as a religious bigot.
Yup...spot on.
God gives us some fairly succinct answers in Romans. One of the easiest to remember is Chapter 6 verse 14 “For sin shall not have dominion over you: for ye are not under the law, but under grace.”
That verse is the 20 second crash course in dispensationalism.