GOD is Unified
Our starting point for any study of character traits or attributes should be to look at the nature of GOD. GOD wants man to imitate His character and nature in every way, so He always provides the target for which we should aim. Unity is no exception. GOD is comprised of three separate spiritual Beings—God the Father; God the Word, who became Jesus Christ, the Son of God; and God the Spirit, referred to in Scripture as the Holy Spirit.
Though these are three separate Beings, and each is God, there is but one GOD. This concept is difficult for humans to explain, because mathematically, one is not equal to three. The only way that Father, Word and Spirit can be one GOD is that they are completely and totally unified. All three Beings are intimately involved in everything that happens, from before the creation and forever, but there has always been one eternal plan. As the apostle Paul wrote:
4 There is one body and one Spirit, just as you were called to one hope when you were called; 5 one Lord, one faith, one baptism; 6 one God and Father of all, who is over all and through all and in all.
(Eph. 4:4-6 NIV)
For more on the unity of GOD, including Scripture references, read: GOD is Unified.
In the Beginning
At the creation, humanity enjoyed a perfect and harmonious relationship with GOD. GOD talked directly with Adam and GOD proclaimed that everything was good. There was unity between GOD and man. Although there was only one man and one woman at the time, they were perfectly unified as well.
When Adam and Eve sinned, eating of the forbidden tree (Gen. 3), everything changed. Man now knew both good, which we were created to know, and evil, which GOD did not create us to know. The unified relationship was broken and man became enemies with GOD (Rom. 5:10). Unquestioned devotion to GOD in the hearts of men was replaced with persistent internal battle between the forces of GOD and the forces of Satan.
We don't even have to leave the Garden in order to see the impact that sin had on unity. When GOD confronts Adam about his sin, Adam immediately blames Eve. Not to be outdone, Eve claims she was tricked by Satan (Gen. 3:11-13). The harmonious relationship between Adam and Eve was temporarily disrupted, and it would never be quite the same as a result of their sin.
Unity Under the Patriarchal Age
With sin having corrupted GOD's perfect creation, what did GOD expect with regard to unity between humans? Scripture doesn't directly answer this question, but we are given enough clues to form a general conclusion. The time between Adam and the Law of Moses is commonly referred to as the Age of Patriarchs. During this time period, GOD spoke directly to certain men, often the head of the family, and gave them direct instructions. Although the vast majority of these conversations probably aren't recorded in the Bible, some are.
First, we see that GOD must have taught men not to kill one another. GOD warned Cain (recorded in Gen. 4:6-7) because He knew Cain's heart was filled with hatred. When Cain killed his brother Abel, GOD punished Cain and told him directly what his punishment would be. It seems clear that GOD gave certain instructions to the patriarchs as to how to live sin-free lives, and to offer sacrifices that were acceptable to GOD. Unfortunately we don't know how detailed and specific GOD's instructions were regarding love and proper relationships.
As time progressed and the human population of earth increased, relationships became more complex and new challenges were introduced. After the worldwide flood, Noah's descendants traveled east to the plain of Shinar. They decided to construct the Tower of Babel, for the purpose of remaining together. It seems that, among other reasons, the Tower of Babel was a human attempt to remain unified.
Interestingly, for whatever reasons, GOD saw that it was best to scatter the people, so He confused their languages where they could no longer understand one another, and scattered them over the whole earth (Gen. 11:1-9). Between this event and the life of Abraham some 300 years later, the families of Noah had grown into various nations of people. With this growth had also come the rise of idolatry, and GOD would begin the process of proclaiming His name throughout the more populous earth, beginning with one man—Abram of Mesopotamia.
After GOD called Abram to leave his home of Ur in the region of Mesopotamia, Abram went to the land of Canaan, along with Sarai his wife and Lot his nephew. In time, there was strife between the herdsmen of Abram and Lot, because they had both become too prosperous to continue living in the same area. Abram resolved the strife with Lot by suggesting they dwell in separate areas.
The first battle between nations that is recorded in scripture is in Gen. 14, where multiple kings fought one another, and Lot was captured in the process (Gen. 14:12). Abram took 318 servants, divided them up and attacked Lot's captors and freed the prisoners. This act of defensive violence was praised by GOD's high priest at the time, Melchizedek, who stated that GOD had delivered these people into Abram's hand.
It is clear from the Old Testament that, while GOD would prefer men to live peacefully towards one another, He acknowledged that there would be conflicts between nations, and GOD supported His people engaging in battle, when the cause was just, as it was with Abram. In fact, GOD would later call the nation of Israel to purge the earth of the Canaanite nations as a means of punishing them for their extreme wickedness and, more specifically, their idolatry.