"Charity never faileth."
We often define charity as Christlike love. Pure charity isn't just like the love of Christ, it is the same love that Christ has. This kind of love truly will never fail, whereas lesser love is weakened by time, distance, disagreement, ingratitude, or betrayal.
Pure charity has been tested to its fullest extent only once, when the Savior performed His Atonement. In that act, Jesus Christ's suffering was infinite, yet he was still kind. Knowing His own power, he could have belittled His provokers, but instead He was as meek as a lamb being led to the slaughter. His desire was to relieve HIs own suffering, yet He sought not His own will. Only once has pure charity proven it can bear all things, and yet endure undiminished.