What does Mark 10:43 mean?
"Servant" is from the Greek root word diakonos. It refers to someone whose duties are to fulfill the desires of another. Diakonos is where we get the word "deacon" from, servants of the church who make sure church resources are distributed to those who need them (Acts 6:1–6).This speaks to the role of a leader in the church, who should become a leader, and the proper motivation of leaders in the church. Often, people take a leadership position because their father was a pastor, they have free time, they are guilted into it, or they happen to be in the right place at the right time. Jesus says the role of a leader in the church is that of a servant: someone who takes care of the needs of another. Granted, servant-leadership often involves impressing upon people what it is they actually need. But it is all for others, not for selfish ambition, pride, or honor.
God interacts with the people of His creation in different ways throughout history. The disciples don't realize it, but they are at the tail end of a 2100–year era wherein God's design was to insulate a family and the nation that came out of that family from the evil world around them. In that context, strong rulers (civil and religious) were needed to represent God's authority and keep the people in line, and a strong military was needed to defend the nation from their enemies.
On the day of Pentecost, that model spiritually shifted from a single nation to a church made of people from all nations. In AD 70, the model disintegrated as the Romans destroyed the temple, along with Jerusalem, and dispersed the Jews. In this new church, members are not born into community, like Jews, but invited (Acts 2:38–40). There is no national identity; there is a spiritual unity (Ephesians 2:11–22). The church cannot rely on the protection and services of a civil authority; they must take care of each other (Acts 2:42–47) while submitting to that secular authority (Romans 13:1–7). In such a vulnerable, disparate group, leaders must be the greatest servants of all.