False.
You would agree that the Gospels, and the content therein, occurred before Paul's conversion, yes? This would lead logically to the events described, such as the Apostles witnessing the Risen Christ, to have occurred before Paul's experience on the road to Damascus. Therefore, to assert that Paul was preaching the Resurrection before the Apostles, is illogical. The Apostles would have known the justification that went with Christ rising. To imply otherwise would go against the teachings and very Words of Christ.
If you wish to argue about "who wrote it first," Matthew and James have the earliest composed writings (being Matthew's Gospel and James). Paul comes next, with his letter to the Thessalonians. But even then, we have Mark, a disciple of Peter, and John, one of the 12 Apostles, also composing accounts of the Gospel. Would it not stand to reason that Peter, John, Mark, Matthew, and James would not have known the justification doctrine that comes as a byproduct of the Resurrection? To answer no would be to give more weight to the writings of Paul. Such ideas are dangerous, as it leads to a devaluing of the Gospels, as well as the other writings, which are divinely inspired. How would the Holy Spirit, being eternally fair and good, give partial truths to the Apostles, but the full truth to Paul? To say that Paul taught it first is to imply that this is the case.
Thus, logically, in accordance to the attributes of Christ, and the evidence put forth in the Gospels and subsequent Scriptures, we can conclude that the Resurrection was declared, as well as, justification, by the Apostles, before Paul.
The apostles did not fully understand what God had accomplished in Jesus Christ until the day of Pentecost, when all was revealed to them by the Holy Spirit, Acts 2. Pentecost gave birth to the New Testament church. Peter preached the Gospel for the very first time, thousands heard and believed, Acts 2:41. The Gospel that Paul received directly from Jesus was more indepth, Galatians 1:11,12.