Unity Under the Law of Moses
As GOD's eternal plan unfolded, He made the descendants of Abraham into a great nation. Under the leadership of Moses, the children of Israel left Egypt and journeyed to Mount Sinai. There, GOD entered into a covenant relationship with them to be their God and they His people. GOD gave Moses a law which governed all aspects of life for the Israelites, including personal, spiritual, social and civil relationships.
The Law of Moses was lengthy and extremely detailed, a necessity for properly explaining GOD's expectations for His people, so GOD began the Law with a simplified overview known as the Ten Commandments (Ex. 20). These 10 instructions were basic and short enough that the Israelites could memorize them, yet broad enough to govern all facets of life when followed.
In time, due to their desire to be like the nations around them, Israel became a kingdom under the rule of a human king. So, under this first covenant, in what ways did GOD expect an Israelite—a citizen of the physical nation of Israel—to be unified?
Jesus would later say that the two most important commands in the Law of Moses were to love GOD with all your heart, mind, soul and strength, and to love your neighbor as yourself (Mark 12:28-34). If an Israelite loved GOD with all their being and treated their neighbor as more important than themselves, then that Israelite would naturally follow the more specific commands, such as not to covet, steal, lie, murder, or commit adultery. Thus, individual Israelites were to be unified with one another under GOD's instructions.
Beyond this, the Israelite was expected to obey the rulers and government officials, pay taxes, and bring the prescribed offerings and sacrifices to the priests. In situations where there were disputes between Israelites, they would go before a judge who would make a ruling. They were unified as citizens of one kingdom, working for a common purpose, living in a land GOD had given them.
Sadly, failure to completely wipe out the Canaanite nations resulted in idolatry creeping in, corrupting and destroying the kingdom of Israel. Because of Solomon's many pagan wives, he was pulled away to serve idols and GOD punished him by splitting the nation of Israel into two peoples. Continued pursuit of idolatry ultimately led to the destruction of the northern nation of Israel and the captivity of the southern nation of Judah.
During Jesus' Earthly Ministry
When Jesus came, He lived by the Law of Moses and He preached the coming of the kingdom of heaven. He gained a substantial following of disciples, twelve of whom He selected as His apostles. Were Jesus' disciples unified while He was on earth? After all, they argued frequently over which of them would be the greatest in the kingdom of heaven (Matt. 18:1-4; Mark 9:33-37; Luke 9:46-48; 22:24-30). And Judas Iscariot eventually betrayed Jesus for money.
I submit to you that the answer is, yes, the disciples were unified. There is a difference between unity and harmony. Harmony speaks to peace within the relationship—the absence of arguing, whereas biblical unity speaks to having a common leader, acceptance of one another, plan and mission. The apostles universally recognized that Jesus was their leader, that they were striving to be like Him as His disciples. Remember, the apostles left everything in order to follow Jesus. Though they had a limited understanding about spiritual matters, including the nature of the kingdom of heaven, and though they argued from time to time, they were not divided.
While Jesus was with them on earth, He taught His disciples a better way. He taught them not to lord it over people as the Gentiles did, but instead to be like little children. Jesus pulled a child from nearby when He said these things, using them as a visual aid to drive home the point. Jesus also led by example, showing them the way of a servant, as He served others' needs daily and as He washed the apostles' feet on the night before His crucifixion (John 13:1-17).
Jesus prayed for His disciples to be unified, just as He and the Father were one (John 17:1-26). He gave Himself up on the cross in order to show the extent of His love for each of us. After His resurrection, Jesus once again served His apostles, making them breakfast by the Sea of Galilee (John 21:1-14). Like all other areas of life, Jesus was the perfect leader.