A Voice from the Past - Clement of Rome

So now, my friends, let us get on resolutely with our warfare under His unerring directions. Think of the men who serve our own commanders in the field, and the prompt and orderly obedience with which they go about their duties. Not all of them are marshals, generals, colonels, captains, or the like; nevertheless, each at his own level executes the orders of the emperor and the military chiefs. For the great cannot exist without the small, nor the small without the great, Every organism is composed of various different elements; and this ensures its own good. Take the body as an instance; the head is nothing without the feet, nor are the feet anything without the head. Even the smallest of our physical members are necessary and valuable to the whole body; yet all of them work together and observe a common subordination, so that the body itself is maintained intact. In Christ Jesus, then, let this corporate body of ours be likewise maintained intact, with each of us giving way to his neighbor in proportion to our spiritual gifts.

Clement of Rome, d. 99 AD, First Epistle, 37, 38 (Early Christian Writings, translated by Maxwell Staniforth, Penguin Books, 1968, pp. 42, 43) 

How can we as Christians come to reflect this kind of unity? What are the obstacles to it?