“Adopt the same attitude as that of Christ Jesus, who, existing in the form of God,did not consider equality with God as something to be exploited. Instead he emptied himself by assuming the form of a servant, taking on the likeness of humanity. And when he had come as a man, he humbled himself by becoming obedient to the point of death—even to death on a cross. For this reason God highly exalted him and gave him the name that is above every name, so that at the name of Jesus every knee will bow—in heaven and on earth and under the earth—and every tongue will confess that Jesus Christ is Lord, to the glory of God the Father. Therefore, my dear friends, just as you have always obeyed, so now, not only in my presence but even more in my absence, work out your own salvation with fear and trembling” (Philippians 2:5-12, CSV).
There’s some difficult yet beautiful theology embedded between the first and last verses of this passage that describe a fundamental element of the Christian lifestyle.
Because of the rhythmic character of the words in vs. 6-11, these verses are considered to be the lyrics from an early Christian hymn. That Jesus “emptied” Himself is a phrase much debated by theologians and prompting various theories of what aspects of deity Jesus gave up through His incarnation.
We may not know the extent of the divine attributes Jesus emptied Himself of by leaving heaven to come to earth, but the end result of this “emptying” was a sort of divine humiliation. And, in Jesus’ case, the scope of the humiliation–from God to human, from heaven to earth–is beyond our perception.
Nevertheless, this passage starts out in vs. 5 by calling upon us to assume the same attitude of humiliation–we call it humility–that Jesus exemplified through His emptying.
Although Jesus co-existed with God, He did not use the power of His co-equality with God to control the human beings He created. Instead, He became human to show them the way to God. In so doing, not only did He abdicate His divine authority but He came as a servant to other human beings. And, as a servant He became obedient even to the point of death on a cross.
Jesus’ emptying demonstrated His divine humiliation. He gave up His position in heaven to take on human nature and serve people and He was faithful to His humiliation even unto death. But, this emptying and humiliation gave way to His great exaltation as Lord over heaven and earth.
Paul exhorts Christians to embrace humility in the same way that was demonstrated by Jesus. We must empty ourselves of self-will to live out God’s will. This emptying of self-will and obedience to God’s will produces humility that enables us to “work out our own salvation with fear and trembling” while on this earth.
Then Jesus said to his disciples, “If anyone wants to follow after me, let him deny himself, take up his cross, and follow me. (Matthew 16:24, CSV)

What does the phrase “under the earth” mean in the original text?
In the context of these verses, the simple answer is that the words of the hymn likely refer to both the living and the dead. In other words there is not a place, location or dimension in the universe where a being exists that will not acknowledge that Jesus is Lord!