There serious problems raised by those who insist that Christ had to do anything beyond the Cross. It goes counter to Scripture. This is excerpted from my article. I took out the parts that are not as relevant for this thread.
WHAT HAPPENED AT THE CROSS?
A closer look at 1st Peter 3:18
"For Christ also has once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:"
Notice these details from the verse:
1. Against those who say He suffered twice (since he "died" twice), the verse teaches Christ "suffered once for sins".
2. His suffering for sin was substitutionary ("the just for the unjust"). As an aside: This proves against the Word of Faith teachers (not the focus of this article) that there was no saving suffering of Christ in Hell.
3. His death (singular) is pointedly said to be "in the flesh", not "in the spirit". If Christ did indeed die spiritually, this verse would seem to have said something at this point. It doesn't. The argument of theirs is an argument from silence.
It is not surprising that we have often recourse to Peter's letter, and to Colossians, in combating this issue. Those letters were written to correct gnostic heresies, and the error facing us here is also a gnostic one. The teaching that Christ died spiritually (or that He died twice) is essentially Gnostic because it isn't part of explicit doctrine, it is (say some) implied. By contrast, all the verses that teach of the Lord Christ's death for us, use outward and visible concepts: Blood, stripes, cross, tree, etc.
5. CHRIST'S SAVING DEATH A PHYSICAL DEATH WITH SPIRITUAL IMPACT
" O foolish Galatians, who has bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ has been evidently set forth, crucified among you? This only would I learn of you, did you receive the Spirit by the works of the law, or by the hearing of faith?"
Apparently, only the image of Christ's physical death (as preached by Paul) was all the Galatians needed to anchor their faith on. It was from this "hearing of faith" that they received the Spirit. I really believe the basis for our faith is as simple as this. We are the ones that complicate it by obscuring details and convoluted speculations. God purposely set forth a simple means, a humbling means, of forgiveness and entrance into life eternal, the simple and shameful cross of Christ.
There are many other verses that could be studied to show the importance of Christ’s physical death, and the saving benefits to the saints that came to us through that death. Another example is Eph. 2:15
“Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of two one new man, so making peace;”
Notice: Jews and Gentiles were alienated from each other (this is one of two estrangements spoken of in this passage, the other being man’s alienation from God). Where were Jew and Gentile brought together? In the spirit of Christ? No, “in His flesh”.
As stated earlier, He is our perfect, but not our total, substitute: He fulfilled all righteousness (Matt. 3:15), but did not partake of all sin. "He tasted death for every man" (Heb. 2:9), but did not sin unto death. Yet that is what merits the penalty of spiritual death! (James 1: 15)