64 From what We have thus far written, and explained, Venerable Brethren, it is clear, We think, how grievously they err who arbitrarily claim that the Church is something hidden and invisible, as they also do who look upon her as a mere human institution possession a certain disciplinary code and external ritual, but lacking power to communicate supernatural life. On the contrary, as Christ, Head and Exemplar of the Church "is not complete, if only His visible human nature is considered..., or if only His divine, invisible nature..., but He is one through the union of both and one in both ... so is it with His Mystical Body" since the Word of God took unto Himself a human nature liable to sufferings, so that He might consecrate in His blood the visible Society founded by Him and "lead man back to things invisible under a visible rule."
Source: Mystici Corporis Christi (Vatican.va)