Without the Resurrection, the Apostle's Preaching Would Be in Vain
There were still many living witnesses of the resurrected Christ in Paul’s time. Most compelling is the testimony of those who walked most closely to Jesus. These were people who saw Jesus suffering, from His sweat and blood agonizing in the garden of Gethsemane, to His betrayal, His unjust trial and crucifixion. They would have had the most to lose for keeping the teachings of Christ alive, and they knew it, hence the running scared after His death. But these former cowards who had abandoned their Master when he needed them most, risked their lives to preach that He lives. All of the apostles, with the exception of John, died a martyr's death. If Jesus had not risen, they wouldn't have risked this fate.