Unity

Jesus said: "Holy Father, keep them in your name, which you have given me, 
that they may be one, even as we are one." (John 17:11b).

The Father and the Son are one. Jesus thinks like God the Father in everything.
If we have seen Jesus, we have seen the Father (John 14:9–11),
see The Order between Jesus and the Father in the Trinity.

Jesus said: "I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and
I in you, that they also may be in us, so that the world may believe that you have sent me. The glory that you have given me I have given to them, that they may be one even as we are one, I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me." (John 17:20–23). 

See also Jesus Farewell Prayer in Jesus, the Way, the Truth and the Life.

That we as Christians are one is important for several reasons (Eph 4:3):

• So the people in the world understand that God the Father has sent Jesus to Earth to give his life for the people, which Jesus did of his free will.
• That God the Father loves believers in the same selfless way that he loves Jesus. We know that God the Father loves Jesus very much, but he also loves believers in the same way (John 17:23).

It's a good and blessing thing when there is unity (Psalm 133). One of the ways to receive blessings in the home and marriage is to stay united. Jesus said: "Every kingdom divided against itself is laid waste, and no city or house divided against itself will stand." (Mat 12:25). One reason we don't have unity is because of pride.

Division and disunity cause serious problems. The Christians in Corinth struggled with disunity (1 Cor 1:10–13), see The Book of Corinthians.

Jesus said: "The kings of the Gentiles exercise lordship over them, and those in authority over them are called benefactors. But not so with you. Rather, let the greatest among you become as the youngest, and the leader as one who serves. For who is the greater, one who reclines at table or one who serves? Is it not the one who reclines at table? But I am among you as the one who serves." (Luke 22:24-27). See Who is the Greatest in the Kingdom of Heaven?

We must not exercise lordship over each other. This goes against hierarchical systems with a strong charismatic leader who decides what everyone can think,
see the last part of The Book of John. We should instead serve each other.
See also the last paragraph in Dealing With Sin in the Church.

If someone is further down in a hierarchical system and expresses a dissenting or different opinion, it may be that someone who feels higher up in that hierarchical system, offends that person and publicly reprimands that person, regardless of whether that person is right or wrong. If the person then adapts to what the leader thinks, unity can be maintained. If the person doesn't do so, the person may be asked to apply to another group that better aligns with the person's view. But this handling of a person isn't biblical. It's pride. "For though the Lord is high, he regards the lowly, but the haughty he knows from afar." (Psalm 138:6).
See examples of good and bad leadership at the end of The Book of John.

The apostle Paul wrote: "There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus." (Gal 3:26–29).

The slave isn't less worthy of God just because he is a slave. The ranking that people do in a group isn't from God. Showing partiality is a sin (James 2:9), see 
The Book of Malachi. See it explained at time 6:17–13:27 in James 2 (Part 1) :1-13.

The apostle Paul wrote: 

• "Love must be sincere. Hate what is evil; cling to what is good. Be devoted to one another in love. Honor one another above yourselves."
  (Rom 12:9–10), see The Golden Rule in The Sermon on the Mount.
• "To speak evil of no one, to avoid quarreling, to be gentle, and to show perfect courtesy toward all people." (James 4:11–12, 5:9, Titus 3:2),
  see Do not judge others in The Sermon on the Mount.
• "So if there is any encouragement in Christ, any comfort from love, any participation in the Spirit, any affection and sympathy, complete my joy by being of the same mind, having the same love, being in full accord and of one mind. Do nothing from selfish ambition or conceit, but in humility count others more significant than yourselves. Let each of you look not only to his own interests, but also to the interests of others." (Phil 2:1–4). 

We need to humble ourselves before each other and put others higher than ourselves, see 3) Blessed are the meek in The Sermon on the Mount. We
shouldn't seek conflict or take positions at the expense of other individuals
(James 4:1, Rom 7:21–23). Jesus rejoices when we live according to this. 
This can be difficult to do with a person considered lower in the social ranking. 

Paul wrote:

• "For as in one body we have many members, and the members do not all have the same function, so we, though many, are one body in Christ, and individually members one of another." (Rom 12:4–5).
• "The parts of the body that seem to be weaker are indispensable, and on those parts of the body that we think less honorable we bestow the greater honor, and our unpresentable parts are treated with greater modesty, which our more presentable parts do not require. But God has so composed the body, giving greater honor to the part that lacked it, that there may be no division in the body, but that the members may have the same care for one another."
  (1 Cor 12:22–25).
• "For consider your calling, brothers: not many of you were wise according to worldly standards, not many were powerful, not many were of noble birth. But God chose what is foolish in the world to shame the wise; God chose what is weak in the world to shame the strong; God chose what is low and despised in the world, even things that are not, to bring to nothing things that are, so that no human being might boast in the presence of God." (1 Cor 1:26–29).
  
• "In that same hour he (Jesus) rejoiced in the Holy Spirit and said: 'I thank you, Father, Lord of heaven and Earth, that you have hidden these things from the wise and understanding and revealed them to little children; yes, Father, for such was your gracious will.'" (Luke 10:21).

See time 17:37–23:57 in Luke 10 (Part 2) :17-24 • Joy, Rejoicing and Blessing.

We as Christians are all members of the same body. For the body to function optimally, the various members need to work together and not fight with each other. The limbs which seem to be the weakest are the more necessary. 

To insult these members when they express an opinion just because they are perceived as the weakest or lowest in the social hierarchy is completely wrong according to the Bible and hurts the body of Christ (Proverbs 18:19).

How a group should work to fulfill this:

• There should be no social ranking in the group. This doesn't prevent different functions in the group, and if necessary to have a biblical leadership, see The Book of Timothy.
• We should submit to one another and not rule over one another.
• We must respect each other and not automatically dismiss the arguments of others, just because we perceive them as lower in the social hierarchy.
• We must treat everyone equally. We should treat other people as we want to be treated ourselves and as we treat our closest friends.
• We must show each other empathy and love (1 Cor 16:14, 2 Thess 1:3),
  see Agape Love at the end of Spiritual Gifts.

Many people follow strong charismatic leaders, which can work effectively
as long as everyone thinks like these leaders and they follow God. However,
I think it's important to let people think for themselves in order to become mature Christians, combined with giving Bible-based teaching that can be tested against God's word in the Bible, see the last part of The Book of John.

Differences of opinion can arise between Christians that aren't related to differences in theology. The reasons for this type of disagreements may be due to different personalities or personal experiences. Paul and Barnabas couldn't agree on whether to take John Mark with them for their second missionary journey. Mark had left them and returned to Jerusalem during their first Missionary Journey. Barnabas wanted to bring him because he wanted to give him a second chance. Paul didn't because he saw this journey as too important to be compromised by bringing Mark with them.
They had a sharp disagreement on this which resulted in them not being able to resolve this issue and hence they split up. Barnabas took Mark with him and sailed away to Cyprus, but Paul chose Silas for his second missionary journey.
(Acts 15:36–40). Paul later wrote about John Mark and everything he had to say about him was very good (2 Tim 4:11, Col 4:10). Their problem was solved.

What to do when believers disagree:

• We must understand that there will be disagreements. 
  But we shouldn't accept people who cause division and create obstacles contrary to central biblical doctrines (Rom 16:17–18). 
• Check our attitudes.
  We should do nothing out of selfish ambition or conceit (Phil 2:3–4).
  We should forgive as the Lord forgave us (Col 3:13),
  see Forgive and You will Be Forgiven.
• Understand the dynamics.
  The situation between Paul and Barnabas was a matter of opinion and  became a matter of separation. If it's not a scriptural, legal, or moral issue, it's probably a matter of opinion. In this case, neither was subordinate to the other. If it's a matter of opinion and we have leaders, we should obey and submit to them (Heb 13:17). This doesn't apply if they do something unscriptural, illegal or immoral. We obey Christ first.
• Respond appropriately.
  In this case, Paul and Barnabas didn't respond appropriately because it was a matter of opinion.
• Seek forgiveness where needed. 
  If we say things or do things that hurt people, we should ask them for forgiveness.
• Separation should be a last resort (1 Cor 1:10).

Much teaching is needed in the congregations about how we should act towards each other and how we should treat each other, in order to achieve unity in a biblical way
(Rom 12:16a, 18, 15:5–6), see time 6:46–9:13 in 1 Peter 3 (Part 2) :8-22.

In the political world, I believe that democratic governance is the best way to govern a country, at least in countries with a high Christian population. It's no coincidence that democracy has emerged in countries with high Christian values. I believe that the judgment system in the Old Testament was a good way to rule Israel, although
it wasn't perfect (Judges 2:18–19), see The Book of Judges.

See also Some stories in the gospels at the end of The Four Gospels.