For Mike McKinley (and many others), it would be Matthew 7:21-23:
Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?’ Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’
So Mike's written a new book, Am I Really a Christian? to help people investigate Christianity. He says,
Let's be honest, churches all over are full of (and empty of!) people who wear the label "Christian" but don't know Jesus and aren't known by him. I wrote "Am I Really a Chrisitan?" in order to help those people see their ongoing need for Jesus and his salvation.
There is also a new website, www.amireallyachristian.com, with answers to common questions like What is the gospel, What is a nominal christian and I'm a good person, is that enough?. There's also a US-only church finder.
You can preorder the book now. It's due out mid-July.