In this sermon on Ephesians 4:9–10 titled “The Drama of Redemption,” Dr. Martyn Lloyd-Jones addresses this difficult passage with the reference to Christ descending below. After mentioning various interpretations of this passage (Christ descending to hell after His death or going to Sheol), Dr. Lloyd-Jones comes to the conclusion that this speaks of Christ’s descent as His coming down to Earth as a man. He did this in order to die on the cross so that all who believed might be saved and come to a true and saving knowledge of God. But to die as a man He had to come down from heaven, be born as a baby, and live as a true human being. Christ did not simply appear to be a man, but the Son of God hid His glory by taking on a true human nature and dying a cruel death upon a cross. It is this sacrificial death that stands at the very heart of true Christianity. If Christ did not die as a man, then there is no salvation for anyone, and all are still in sin. But God’s word tells that Christ truly died so that all may have everlasting life with Him.

http://www.mljtrust.org/sermons/the-drama-of-redemption/