How can I witness to a person who thinks the gospel is just my opinion?

I would do my best to direct people first to the Bible and say, "Now whether you believe the Bible or not, you need to know that I'm not making these things up. They are here."

This is why I try to preach in an expository way and point people to the exact words of Scripture: because Scripture is amazing. The Bible has a kind of unique authority. It is a sword that penetrates to the division of soul and spirit, bone and marrow. It's not like John Piper's words. It has its own divine claim.

So I want to direct people to it and say, "OK, in your mind what I'm saying may just be my opinion, but at least see here where I'm getting it from the Bible." And then, secondly, as a stage beneath that I would say, "In order to give the Bible credence I would just invite you to consider the portrait of Jesus as it is laid out in Matthew, Mark, Luke, and John, and the rest of the New Testament. And see whether or not your encounter with Jesus Christ will vindicate him as true and a faithful witness to these things in your life."

But if they won't go there, that is, to look at the Bible and to look at Jesus with me, then all I have left to do for them is to pray for them as they leave.