Question: "If Jesus was God, why did He say 'The Father is greater than I' in John 15:1?"
Answer: The phrase “the Father is greater than I” (John 14:28) was spoken by Jesus during the upper room discourse, and the greater context is the promising of the Holy Spirit to the disciples after Jesus’ resurrection. Jesus says repeatedly that He is doing the Father’s will, thereby implying that He is somehow subservient to the Father. The question then becomes how can Jesus be equal to God when by His own admission He is subservient to the will of God? The answer to this question lies within the nature of the incarnation.
During the incarnation, Jesus was temporarily “made lower than the angels” (Hebrews 2:9), which refers to Jesus’ status. The doctrine of the incarnation says that the second Person of the Trinity took on human flesh. Therefore, for all intents and purposes, Jesus was fully human and “made lower than the angels.” However, Jesus is fully divine, too. By taking on human nature, Jesus did not relinquish His divine nature—God cannot stop being God. How do we reconcile the fact that the second Person of the Trinity is fully divine yet fully human and by definition “lower than the angels”? The answer to that question can be found in Philippians 2:5-11. When the second Person of the Trinity took on human form, something amazing occurred. Christ “made himself nothing.” This phrase has generated more ink than almost any other phrase in the Bible. In essence, what it means is that Jesus voluntarily relinquished the prerogative of freely exercising His divine attributes and subjected Himself to the will of the Father while on earth.
Another thing to consider is the fact that subservience in role does not equate to subservience in essence. For example, consider an employer/employee relationship. The employer has the right to make demands of the employee, and the employee has the obligation to serve the employer. The roles clearly define a subservient relationship. However, both people are still human beings and share in the same human nature. There is no difference between the two as to their essence; they stand as equals. The fact that one is an employer and the other is an employee does nothing to alter the essential equality of these two individuals as human beings. The same can be said of the members of the Trinity. All three members (Father, Son and Holy Spirit) are essentially equal; i.e., they are all divine in nature. However, in the grand plan of redemption, they play certain roles, and these roles define authority and subservience. The Father commands the Son, and the Father and the Son command the Holy Spirit.
Therefore, the fact that the Son took on a human nature and made Himself subservient to the Father in no way denies the deity of the Son, nor does it diminish His essential equality with the Father. The “greatness” spoken of in this verse, then, relates to role, not to essence.