Psalms 133:1-3 (KJV)
1 A Song of degrees of David. Behold, how good and how pleasant it is for brethren to dwell together in unity!
2 It is like the precious ointment upon the head, that ran down upon the beard, even Aaron’s beard: that went down to the skirts of his garments;
3 As the dew of Hermon, and as the dew that descended upon the mountains of Zion: for there the LORD commanded the blessing, even life for evermore.
What a promise if we the church are in unity. The LORD will bless this unity with His  
When Jesus Christ prayed, before going to the cross, He prayed for unity (oneness) five times in that prayer. This means being one with each other and GOD is so important. 
John 17:11 (KJV) And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are [ONE].
John 17:20-23 (KJV)
20 Neither pray I for these alone, but for them also which shall believe on me through their word;
21 That they all may be one [TWO]; as thou, Father, art in me, and I in thee, that they also may be one in us [THREE]: that the world may believe that thou hast sent me.
22 And the glory which thou gavest me I have given them; that they may be one, even as we are one [FOUR]:
23 I in them, and thou in me, that they may be made perfect in one [FIVE]; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me.
Yes, we are one because we are the body of Christ because we believe in Jesus Christ. This is unity, that I believe can be considered, in a spiritual sense just like our spirit is saved when we are born again. But our soul/mind, after that, is being transformed renewed day by day and finally when the LORD comes to take us home we will have a new body. So, I believe, it is with the church too. Now as the LORD is transforming us the body of Christ, we are exhorted to keep the unity in faith.  
Ephesians 4:1-6 (KJV)
1 I therefore, the prisoner of the Lord, beseech you that ye walk worthy of the vocation wherewith ye are called,
2 With all lowliness and meekness, with longsuffering, forbearing one another in love;
3 Endeavouring to keep the unity of the Spirit in the bond of peace.
4 There is one body, and one Spirit, even as ye are called in one hope of your calling;
5 One Lord, one faith, one baptism,
6 One God and Father of all, who is above all, and through all, and in you all.
Jesus made it possible for us to come to that unity in faith, by His Word, His Holy Spirit  and also raised up ministers 
Ephesians 4:11-13 (KJV)
11 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
12 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
13 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
This unity is demonstrated by how we treat each other as we saw in Ephesians verses 1 to 3.
Let us aim to be people who seek unity in the body of Christ through love without compromising the undeniable truth of the gospel and who Jesus Christ is. Through this demonstration of unity we will see souls saved and people grow to become more like Jesus (for there the LORD commanded the blessing, even life for evermore).
John 13:34-35 (KJV)
34 A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.
35 By this shall all men know that ye are my disciples, if ye have love one to another.