"Tepid souls shall become fervent."
None of us appreciate half-measures and half-heartedness in others when it comes to things we value greatly. We put our 100% into something, be it family life, our jobs or some cause, and we see many others dragging their feet. How must Jesus feel when He sees the multitude of souls which are neither hot nor cold but lukewarm. We know how He must feel, for in Revelations it is said that God will vomit them out of His mouth. God, who loves us infinitely, and gives all of Himself to us, will not be happy with something less than all of ourselves. Love is paid with love.
Spiritual lukewarmness is not the dry spells in feeling or fervor that good souls often experience when in prayer. Those dry spells are often the very thing God wants us to undergo, so that we do all things for genuine love of God, not because doing the good gives us an emotional high.
Tepidity is a real attitude of indifference and carelessness; of going through the motions and moreover not being concerned that one is merely going through the motions. It is to walk by the edge of a cliff, and not even realize it.
But the Sacred Heart of Jesus is a burning furnace ablaze with love. Just as fire melts away the outer frozen shell first, till the inner flesh is exposed and enjoys the warmth of the fire, the love of Jesus first destroys the hardened outer shell of the tepid soul, attacking its indifference and apathy, jolting the soul out of its stupor with remembrances of eternal damnation versus the eternal joys of heaven. It exposes the soul's longing for love, and directs that longing to the perfect love of Jesus. Then the human heart takes on the warmth of the Sacred Heart to which it is exposed.