Jesus didn't say that all men would know we are His disciples by our doctrine, our rituals, our hatred for sin, or even by the way we express our love for God. He said very clearly, that the one characteristic that would cause the world to identify us as His followers, is our love, one for another.

This same night, the Lord prayed to His Father using this same thought saying, "That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me" (John 17:21). The only way that Christ's body will be one as the Father and Jesus are One, is through God's kind of love.

Unity of believers, that can only come through a genuine God-kind of love, is the greatest tool for evangelism that the church has or will ever have, according to Jesus. 

The early church didn't have the massive organizational structures that we see today or the ability to travel anywhere in the world in just a matter of hours. They certainly did not come close to spending as much money, in proportion to us, to spread the gospel. 

The pagans of Thessalonica said of Paul and his companions, "These that have turned the world upside down are come hither also" (Acts 17:6). They had evangelized the known world in less than thirty years.

Before we can ever fulfill the great commission of Matthew 28:19-20, there must be a revival of love in the church, where doctrine and ritual take a "back seat" to love for one another.