with all lowliness and gentleness, with longsuffering, bearing with one another in love,
endeavoring to keep the unity of the Spirit in the bond of peace."
Ephesians 4:1-3 (NKJV)
In the New Testament, there is only one book that actually records the history of the Church, that is the Book of Acts. One thing we see repeated again and again, in fact, it is listed six times, is how the early believers were "in one accord". This means that they had the same focus, the same goals, and this is talking about unity. The Early Church had a great deal of success, and one of the key factors was unity. The one thing that is important for unity is that there is a common element that the focus and goals are centered around. The thing the Early Church was focused on, after Jesus, of course, was the Word of God.
The only way for Christians to truly have unity is that there has to be an agreement that God's Word is our basis for what we believe. If we leave what we believe as Christians open to any other factor outside of the Bible, which is where we actually learn about Jesus and what He did for us, there will not be harmony in the Body of Christ. To have unity we must keep our focus on the Word and we must walk in love with others in areas of minor issues of style and personal preference that are not clearly addressed by God's Word. God's Word needs to be our standard for what we believe and it is the only focal point that will bring and maintain unity in the Body of Christ.