Christ's sacrifice on the cross.
Christ's resurrection demonstrates His sacrifice was acceptable to God for the forgiveness of sins.
This is basic Christianity 101
explaining and demonstrating that the Christ
had to suffer
and rise again from the dead, and
saying, “This Jesus whom I preach to you is the Christ.” Acts 17:2
John Gill on 1 Cor 15:17
ye are yet in your sins: in a state of nature and unregeneracy, under the power and dominion of sin, being neither regenerated nor sanctified; for regeneration is owing to the resurrection of Christ from the dead, and is a branch of the power, virtue, and efficacy of it: but if Christ is not risen, there never was, is, or will be any such thing as regeneration and sanctification; things, if ever wrought by the Spirit, are done by him in virtue, and in imitation of the resurrection, as well as the death of Christ: moreover, if Christ is not risen, his people are under the guilt of their sins; there is no expiation nor remission of them, nor justification from them; for though he was delivered as a sacrifice to atone for their offences, and his blood was shed to obtain the forgiveness of their sins, yet he must be raised again for their justification, and be exalted as a Prince and a Saviour, as to give repentance, so remission of sins, or they will never enjoy these blessings; for notwithstanding his sufferings and death, if he lies under the power of the grave, they must remain under the power and guilt of sin, and be liable to everlasting punishment for it.
Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you. John 16:7
Who by him do believe in God, that
raised him up from the dead,
and gave him glory; that your faith and hope might be in God. 1 Peter 1:21
This Jesus hath God raised up, whereof we all are witnesses. Therefore being by the right hand of God exalted,
and having received of the Father the promise of the Holy Spirit,
he hath shed forth this, which ye now see and hear. Acts 2:32,33
Did that actually take place? How important to the shedding forth of the Spirit was the resurrection out of the dead? How was a dead Jesus going to send the Spirit?
and this he said of the Spirit, which those believing in him were about to receive; for not yet was the Holy Spirit, because Jesus was not yet glorified. John 7:39