Question: : Why did Jesus’ penalty for “sin” need to include physical and emotional pain – why not just spiritual separation from God?
Answer: First of all, I agree with you that Jesus’ penalty payment had to include physical and emotional pain, or God would have found another way. The question is, why couldn’t he have simply (is that possible?) have separated himself from Jesus spiritually without all the suffering of the cross?
I cannot imagine, actually, that Jesus could have experienced spiritual separation from the Father without at the same time experiencing physical and emotional pain. That is the definition of hell, to be separated from God and the resulting pain such separation causes.
But I think the other thing to consider is that if Jesus would only have experienced spiritual separation and there was no way to observe his suffering, we would not have had the assurance we need that God has fully taken on our pain, has been accustomed to pain all our existence, and embraced that pain in such a way that He redeemed the experience. We suffer so much exactly because we are made in God’s image, but we sometimes think that He is somehow exempt from pain. The cross has forever put that myth to rest. We still cause God pain when we “grieve the Holy Spirit” (Ephesians 4:30). We can come to see that God is in pain when we go through physical and emotional pain ourselves.