One body - Ephesians 4

Getting this right will help you have your Bible doctrine correct in other areas of study.

The one body is the assembly of scripturally baptized believers that gather together at a specific location.  It is called the church.  Preaching takes place, hymns and songs are sung, an offering is taken, and the two ordinances are observed.  Its members have the responsibility of preaching the gospel to every creature. 

Now for those that have trouble with that definition, lets look at the scriptures.

Col 1:18
And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.

And the church is shown clearly to be a visible, local assembly in
I Timothy 3:15
But if I tarry long, that thou mayest know how thou oughtest to behave thyself in the house of God, which is the church of the living God, the pillar and ground of the truth.

So the body is the church and the church is a place where your behavior is addressed in the scriptures.  Therefore the one body is the N.T. church. 

In case you were wondering, there is no such thing as the universal, invisible, body or church.  For there is only one body, we have just defined it, don't go trying to find a second body, the Bible said there is only one.