D.A. Carson answers the question of why the resurrection of Jesus Christ is so important, and not simply a matter of speculation or opinion. In his commentary on the Gospel of John (which has been invaluable to me as I preach through John), he writes,
For John, as for all the early Christians, the resurrection of Jesus was the immutable fact upon which their faith was based; and their faith in large part depended on the testimony and transformed behaviour of those who had actually seen the resurrected Jesus. Their Master was not in God’s eyes a condemned criminal; the resurrection proved that he was vindicated by God, and therefore none less than the Messiah, the Son of God he claimed to be. The culminating faith that brings the disciples out of the era of the Mosaic covenant and into the era of the saving sovereignty of God mediated through the Son is based on the sheer facticity of the resurrection (20:8, 24-29) – or, better put, such faith trusts Jesus as resurrected Lord. Nor is John alone on the non-negotiability of the resurrection, for Paul writes, “And if Christ has not been raised, our preaching is useless and so is your faith. More than that, we are then found to be false witnesses about God that He raised Christ from the dead…And if Christ has not been raised, your faith is futile; you are still in your sins” (1 Cor. 15:14-17).