14 That the Church is a body is frequently asserted in the Sacred Scriptures. "Christ," says the Apostle, "is the Head of the Body of the Church." If the Church is a body, it must be an unbroken unity, according to those words of Paul: "Though many we are one body in Christ." But it is not enough that the Body of the Church should be an unbroken unity; it must also be something definite and perceptible to the senses as Our predecessor of happy memory, Leo XIII, in his Encyclical Satis Cognitum asserts: "the Church is visible because she is a body. Hence they err in a matter of divine truth, who imagine the Church to be invisible, intangible, a something merely "pneumatological" as they say, by which many Christian communities, though they differ from each other in their profession of faith, are untied by an invisible bond.
Source: Mystici Corporis Christi (Vatican.va)