The Mark of the Christian by Francis Schaeffer
I believe I paid under a dollar for this book. It is only 35 pages long. Though cheap and short, it addresses the love we should have for one another in John 13 in the most comprehensive and intelligible way I have ever seen. Francis minces no words.
He boldly addresses our need to weigh if those who claim to be in the faith truly are. One who is a false brother or a "Christian" by religious affiliation only cannot receive the Spirit love that we are endowed to give one another. The love between true Christians is a unique gift that we are only able to give and receive because the Spirit within us enables us to do so. If we are in Christ and refuse to give this love to a fellow believer, the world has every right to judge us as not being in Christ. To not love is to refuse His mark.
John 17:21 tells us that we should be one as Christ and the Father are one. To refuse to be unified as one tells the world a deep message according to this verse. It tells the watching world that the Father did not send the Son. Schaeffer again addresses the ideas of false unity that are propagated throughout the church. Beware of being unified in anything but Christ.
"It is not a mushy, gooey, romantic love, but one that is practical and that a watching (sinful) world can see." Christian love is deep and selfless. What are we showing to the watching world?