How Do We Know We Have the Original Gospel?

A comment question I hear among skeptics (and among believers!) is, "How do we know that we have the original Gospel?" Often, that question is not worded in such open, kind, inquisitive terms - and more often it is a negative statement instead of a question. But the question is still there.

The answer to the question requires at least a book-length answer, but let me give you a few things to consider in response:

1. We have extremely early evidence that the Gospel we preach is what the apostles preached. For example, 1 Cor 15:1-5 explains the same basic core of what the Gospel is as what we believe - atonement for sin in the death, burial, and resurrection of Jesus. 1 Corinthians is easily shown to be one of the earliest Christian documents, evidence that what we believe to be the core of the Gospel has been the core since very early in church history (before A.D. 50). But also consider that 1 Cor 15:1-5 appears to be quoting a common saying. If that's true, then the core of the Gospel shows up even earlier than 1 Corinthians. It takes a while for common sayings to become common!
2. There is terrific evidence that every book of the New Testament was written in the 1st Century, and they all agree on what the Gospel is. The book of Acts is particular good about boiling the Gospel down to it's primary content.
3. There is no other 1st Century evidence of any other gospel. (There are "other gospels" that appear in the 3rd Century, but nothing from the first.) So, the only evidence we have from the 1st Century is consistent on what the core of the Gospel is. Anyone who believes we don't have the original Gospel has to explain why all the 1st Century evidence points only in one direction. I've heard theories, but they require more faith and speculation than what we're accused of.
4. There are Roman and Jewish documents from the 1st Century that are hostile against Christianity, and yet they confirm that the main doctrine of Christians is that they believe in the resurrection of Christ. They don't agree with that belief, but they affirm that this is what Christians believed. These documents by no means want to promote Christianity - even hostile primary sources affirm the content of the Gospel!
5. The apostles preferred death over denying the resurrection of Christ. That's how core this belief has been from the beginning. 
6. Jesus wasn't killed for being a controversial religious figure, or for being a controversial teacher, or for being a political activist. He was killed for claiming to be equal to God. The very reason for killing Him (from the human perspective) affirms that His claims to divinity were core to His message. If He didn't claim to be equal to God, He likely would have lived to a ripe, old age.

The list goes on, but this gives you a taste of why we have a strong defense against any charge that we do not possess the original Gospel. Be emboldened by the facts - they are always on the side of Truth. In my experience, the more we dig, the more reasons we have to believe, not less.