5 You know well, and We have frequently reminded you of it, nothing was so often and so carefully inculcated on His disciple by Jesus Christ as this precept of mutual charity as the one which contains all others. Christ called it the new commandment, His very own, and desired that it should be the sign of Christians by which they might be distinguished from all others; and on the eve of His death it was His last testament to His disciples to love one another and thus try to imitate the ineffable unity of the three Divine Persons in the Trinity. "That they may be one as we also are one . . . that they may be made perfect in one"
Source: Pacem, Dei Munus Pulcherrimum (Vatican.va)