17 The faithful would not, however, so completely and advantageously satisfy these duties as is fitting they should were they to enter the field as isolated champions of the faith. Jesus Christ, indeed, has clearly intimated that the hostility and hatred of men, which He first and foremost experienced, would be shown in like degree toward the work founded by Him, so that many would be barred from profiting by the salvation for which all are indebted to His loving kindness. Wherefore, He willed not only to train disciples in His doctrine, but to unite them into one society, and closely conjoin them in one body, "which is the Church,"(17) whereof He would be the head. The life of Jesus Christ pervades, therefore, the entire framework of this body, cherishes and nourishes its every member, uniting each with each, and making all work together to the same end, albeit the action of each be not the same.(l8) Hence it follows that not only is the Church a perfect society far excelling every other, but it is enjoined by her Founder that for the salvation of mankind she is to contend "as an army drawn up in battle array."(19) The organization and constitution of Christian society can in no wise be changed, neither can any one of its members live as he may choose, nor elect that mode of fighting which best pleases him. For, in effect, he scatters and gathers not who gathers not with the Church and with Jesus Christ, and all who fight not jointly with him and with the Church are in very truth contending against God.(20)
Source: Sapientiae Christianae (Vatican.va)