Original post by Sean McDowell here. Used with permission.
To be a Christian today is to believe in the resurrection of Jesus. But what about the first Christians? Could belief in the resurrection have been a late church invention? If so, then Easter celebration is deeply misguided and Christians ought to reject the evidence for faith.
Critics often claim that there were a variety of “Christian” beliefs in the first and second centuries—some that embraced the resurrection of Jesus and others that rejected it. The resurrection party happened to “win,” and so contemporary Christians accept it.
The problem with this claim is that there is no early Christianity apart from belief in the resurrection.
Let me say it again—The earliest records we have all indicate that belief in the resurrection of Jesus was at the heart of the Christian faith.
Consider four points:
1. Early Christian Creeds:
Creeds are verbal proclamations that circulated before their inclusion in the New Testament (e.g., Romans 1:3-4, 1 Peter 3:18). They give us a glimpse into the earliest Christian beliefs. Perhaps the oldest creed comes from 1 Corinthians 15:3-5:
For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures, and that he appeared to Cephas, then to the twelve.
Notice two things.
First, Paul passes on a tradition that he had previously been given. Given the formulaic structure of this passage, most scholars agree that Paul is passing on material he received.
Second, the resurrection is of “first importance” for the faith.
2. Early Christian Preaching:
The book of Acts records the beginning and expansion of the church. Resurrection is mentioned in most of the speeches, which make up roughly one-third of the book. In the first speech in Acts, Peter describes how God appointed Jesus to do wonders but he was killed by lawless men, and yet “God raised him up, loosing the pangs of death, because it was not possible for him to be held by it” (Acts 2:24).
3. Early Letters of Paul:
The epistles of Paul are filled with references to the resurrection. N.T. Wright observes:
Squeeze this letter [Romans] at any point, and resurrection spills out; hold it up the light, and you can see Easter sparking all the way through. If Romans had not been hailed as the great epistle of justification by faith, it might easily have come to be known as the chief letter of resurrection. 
4. Early Church Fathers:
Resurrection was a central theme for many of the believers shortly after the apostles. Affirmations of the resurrection can be found in Ignatius Letter to the Magnesians 11, Polycarp Letter of Polycarp to the Philippians 1.2, 1 Clement 42:3, and the Letter of Barnabas 5:6.
The centrality of the resurrection can be seen in the earliest Christians creeds, the first written documents, the first preaching, and in the apostolic fathers.
There simply is no record of early Christian faith divorced from the resurrection.
The resurrection of Jesus was not a late church invention. It was the heart of the earliest Christian faith and proclamation.
Sean McDowell, Ph.D. is a professor of Christian Apologetics at Biola University, best-selling author, popular speaker, part-time high school teacher, and the Resident Scholar for Summit Ministries, California. Follow him on Twitter: @sean_mcdowell and his blog: seanmcdowell.org.
 N.T. Wright, The Resurrection of the Son of God (Minneapolis, MN: Fortress Press, 2003), 241.