6 But He, indeed, Who made this one Church, also gave it unity , that is, He made it such that all who are to belong to it must be united by the closest bonds, so as to form one society, one kingdom, one body - "one body and one spirit as you are called in one hope of your calling ( Eph . iv., 4). Jesus Christ, when His death was nigh at hand, declared His will in this matter, and solemnly offered it up, thus addressing His Father: "Not for them only do I pray, but for them also who through their word shall believe in Me...that they also may be one in Us...that they may be made perfect in one" ( John xvii., 20-21 23). Yea, He commanded that this unity should be so closely knit and so perfect amongst His followers that it might, in some measure, shadow forth the union between Himself and His Father: "I pray that they all may be one as Thou Father in Me and I in Thee" ( Ibid . 21). Agreement and union of minds is the necessary foundation of this perfect concord amongst men, from which concurrence of wills and similarity of action are the natural results. Wherefore, in His divine wisdom, He ordained in His Church Unity of Faith ; a virtue which is the first of those bonds which unite man to God, and whence we receive the name of the faithful - "one Lord, one faith, one baptism" (Eph. iv., 5). That is, as there is one Lord and one baptism, so should all Christians, without exception, have but one faith. And so the Apostle St. Paul not merely begs, but entreats and implores Christians to be all of the same mind, and to avoid difference of opinions: "I beseech you, brethren, by the name of our Lord Jesus Christ, that you all speak the same thing, and that there be no schisms amongst you, and that you be perfect in the same mind and in the same judgment" (I Cor . i., 10). Such passages certainly need no interpreter; they speak clearly enough for themselves. Besides, all who profess Christianity allow that there can be but one faith. It is of the greatest importance and indeed of absolute necessity, as to which many are deceived, that the nature and character of this unity should be recognized. And, as We have already stated, this is not to be ascertained by conjecture, but by the certain knowledge of what was done; that is by seeking for and ascertaining what kind of unity in faith has been commanded by Jesus Christ.
Source: Satis Cognitum (Vatican.va)