That is, having great care bestowed upon it; for the body must not be put together anyhow, but with exceeding art and nicety, since if it gets out of place, it is no longer. So that each must not only be united to the body, but also occupy his proper place, since if you shall go beyond this, you are not united to it, neither do you receive the Spirit. Do you not see, that in those dislocations of the bones which take place in any accident, when a bone gets out of its proper place and occupies that of another, how it injures the whole body, and oftentimes will produce death? So that sometimes it will be found to be no longer worth preserving. For many in many cases will cut it off, and leave a void in its place; because everywhere what is in excess is an evil. And so again with the elements, if they lose their proper proportion and be in excess, they impair the whole system. This is the meaning of the being “fitly framed and knit together.” Consider then of how vast importance it is, that each should remain in his own proper place, and not encroach on another which in nowise appertains to him. You put the members together, He supplies them from above. For as there are in the body such recipient organs, as we have seen, so is it also with the Spirit, the whole root or source being from above. For example, the heart is the recipient of the breath, the liver of the blood, the spleen of the bile, and the other organs, some of one thing, others of another, but all these have their source from the brain. So also has God done, highly honoring man, and being unwilling to be far from him, He has made Himself indeed the source of his dependence, and has constituted them fellow-workers with Himself; and some He has appointed to one office, and others to another. For example, the Apostle is the most vital vessel of the whole body, receiving everything from Him; so that He makes eternal life to run through them to all, as through veins and arteries, I mean through their discourse. The Prophet foretells things to come, while He alone orders the same; You put the members together, but He supplies them with life, “For the perfecting of the saints, for the work of the ministry.” Love builds up, and makes men cleave one to another, and be fastened and fitted together.
Moral. If therefore we desire to have the benefit of that Spirit which is from the Head, let us cleave one to another. For there are two kinds of separation from the body of the Church; the one, when we wax cold in love, the other, when we dare commit things unworthy of our belonging to that body; for in either way we cut ourselves off from the “fullness of Christ.” But if we are appointed to build up others also, what shall not be done to them who are first to make division? Nothing will so avail to divide the Church as love of power. Nothing so provokes God's anger as the division of the Church. Yea, though we have achieved ten thousand glorious acts, yet shall we, if we cut to pieces the fullness of the Church, suffer punishment no less sore than they who mangled His body. For that indeed was brought to pass for the benefit of the world, even though it was done with no such intention; whereas this produces no advantage in any case, but the injury is excessive. These remarks I am addressing not to the governors only, but also to the governed. Now a certain holy man said what might seem to be a bold thing; yet, nevertheless, he spoke it out. What then is this? He said, that not even the blood of martyrdom can wash out this sin. For tell me for what do you suffer as a martyr? Is it not for the glory of Christ? Thou then that yieldest up your life for Christ's sake, how do you lay waste the Church, for whose sake Christ yielded up His life? Hear what Paul says, “I am not meet to be called an Apostle, because I persecuted the Church of God and made havoc of it.” This injury is not less than that received at the hands of enemies, nay, it is far greater. For that indeed renders her even more glorious, whereas this, when she is warred upon by her own children, disgraces her even before her enemies. Because it seems to them a great mark of hypocrisy, that those who have been born in her, and nurtured in her bosom, and have learned perfectly her secrets, that these should of a sudden change, and do her enemies' work.
Source: Homilies on Ephesians (New Advent)