Which Seven Dangers to Human Virtue relate the following Gandhi quote: "It is easy enough to be friendly to one's friends. But to befriend the one...

In my mind, Gandhi's quote specifically warns of the dangers of a life without moral conviction.

The underlying idea of Gandhi's quote is the need to live a life with conviction in one's beliefs.  Gandhi believes that the fibers of our belief system are tested when we hold onto them in the most trying of conditions. True beliefs are not tested when times are good, or when we are in the company of like-minded individuals. In these settings, our values are not really tested.  Real belief is seen when we hold on to them in the most pain-ridden of moments or when we are in the company of those who challenge them in the most intense ways.  This would mean that our beliefs are most evident when we befriend our enemies, the people who cast themselves in opposition to us.  

In this light, Gandhi's quote connects to the blunder of embracing religion without sacrifice.  When we befriend our enemy, it is sacrifice.  We are putting ourselves in some level of harm's way, sacrificing our comfort.  When we sacrifice the life of comfort in the name of our beliefs and still can hold onto them with passion, Gandhi would say that we understand the power of sacrifice that is the test of true religion.

Gandhi also warned of the colossal blunder of living a political life without principle.  In befriending our enemies and embodying "the quintessence of true religion," Gandhi presumes that we do so through political and spiritual principles.  These principles enable us to befriend an enemy, but not negotiate or vitiate our value systems. The fervor of our principles is tested.  Gandhi's quote presumes that our principles guide our political actions, and not the other way around.