And if Christ be not raised, your faith is vain; ye are yet in your sins. Then they also which are fallen asleep in Christ are perished. If in this life only we have hope in Christ, we are of all men most miserable. But now is Christ risen from the dead, and become the firstfruits of them that slept. 1 Corinthians 15:17-20
Three days after dying on the cross, Jesus was raised from the dead. His resurrection proved his deity and validated his substitutionary atonement for our sins. If there is no resurrection, then Jesus did not die for our sins, only for his own. The validity of Christianity rests on the truth of the resurrection.