"By this shall all men know you are my disciples" [John 13.35 on loving one another]
"The importance of this love perspective is further reinforced by Christ's words in 13:35. The one characteristic that he isolates as bearing testimony to the world about the reality of the Christian faith is brotherly love. Not right doctrine, nor denominational creeds, nor persuasive preaching, nor impressive sanctuaries, nor elaborate social programs, nor vast numbers, but genuine and discernible love between believers. In the Reformed tradition the historical 'marks' of a 'true church' are identified as: (1) the Word preached, (2) the ordinances properly administered, and (3) discipline practiced. But we can have all these 'marks' and yet miss the one identifying 'mark' that Christ says is the only one that matters. Without love, all else is in vain (1 Cor. 13:1-3). As his followers, we need the love of our Lord as described in John 13:34 more than anything else."
-Jon Zens, This is My Beloved Son, Hear Him!, 13