John 13:3-5 (New Living Translation)
"3 Jesus knew that the Father had given him authority over everything and that he had come from God and would return to God. 4 So he got up from the table, took off his robe, wrapped a towel around his waist, 5 and poured water into a basin. Then he began to wash the disciples’ feet, drying them with the towel he had around him."
How was Jesus, who was equal to God, able to empty himself and serve in such a humble way, washing the feet even of those who would abandon and betray him? In the passage above we see that Jesus knew who he was. He knew where he came from and where he was going. Because of this he did not have to prove himself in any way. He did not have to put on a show for anyone because he was not seeking anyone else's approval. It was the knowledge of the Father's approval and the security in his identity as coming from the Father that gave him the ability to serve in humility.
The same is true of us. Ephesians 5:1-2 says "1Be imitators of God, therefore, as dearly loved children 2and live a life of love, just as Christ loved us and gave himself up for us as a fragrant offering and sacrifice to God."
Our standing as God's children through the life of Christ in us allows us also to shed our defenses and pretenses and to serve and love in humility like Christ did. When we know who we are (a.k.a. God's child, fully approved, accepted and loved, given authority by Christ) we have no need to prove ourselves. We do not need to push others down so that we can appear better or more important than them, because our identity is not formed by our success in this world or by the perception of others but by the knowledge and reality of our standing before God. We know who we are. We know who our Maker is. We know where we are going, and that this is not the end. We are free to serve in the most "humiliating" ways without ourselves being in some way soiled or devalued. Rather, by serving in these ways we are following in the footsteps of Christ, the fullness of God, who did not consider equality with God as something to be grasped, but emptied himself as a servant and gave himself up willingly for us (see Philippians 2:6-8).