Medieval illustration of Pentecost from the 12th-century Hortus deliciarum of Herrad of Landsberg (details)
From Saint Augustine:
And so, where the Apostle speaks of it as a body, let us not think of it as a dead body without life. One body, he says. But, I ask you, is this a living body? It is living. By what does it live? By one spirit. And one Spirit. Be watchful therefore, brethren, within our own body; and grieve for those who are cut off from the Church. As long as we live, while we are in our senses, let all members fulfil their duties among our own members. Should one member suffer anything, let all the members suffer with it (I Cor. xii. 26). Yet, though it may suffer, because it is in the body, it cannot die. For what does to die mean but to lose the spirit? Now if a member be cut off from the body, does the soul follow it? It can still be seen what member it is: it is a finger, a hand, an arm, an ear; besides substance, it has form; but it has no life. So is it with a man separated from the Church. Seek if he has the sacrament. You learn he has. Look for baptism. You find it. The creed? You find it. This is the outward form; but unless inwardly you live by the Spirit, in vain do you glory in the outward form.