When Jesus cried, ‘It is finished,’ He was not referring to the whole plan of salvation as being completed. He still had to descend into the lower parts of the earth and lead the captives out (Eph. 4:8-9), as well as come back from the dead and ascend to the Father to make intercession for us (Heb. 7:25).
Paul made it very clear in 1 Corinthians 15:14 and 17, that if Jesus did not rise from the dead, then our faith is vain and we are yet in our sins.
Why is the resurrection so important? The resurrection of Jesus makes Christianity different from religion because we are dealing with a real, live person, not just principles. Christianity is fellowship with a real person who is alive.
Also, forgiveness of sins is not obtained by a doctrine, but by Christ Himself. If Jesus did not rise from the dead, He cannot give salvation to us. However, He did rise from the dead and now He lives forever to make intercession for us (Heb. 7:25).
It is Christ living in us that gives us power to live a victorious life. If there was no resurrection, there would be no power.
The resurrected life of Jesus is the guarantee of our physical regeneration.
Because Jesus conquered death, the fear and sting of death have been removed for us. This produces great comfort for those who believe.
So, faith in the present ministry of Jesus is essential to true Christianity, and would be impossible if Jesus was not alive. Without Jesus being alive, Christianity would be just another dead religion.