What is the best 'cure' for spiritual lukewarmness?
To be cold, distant, and indifferent is unacceptable. Yet, such type of individual simply does not know any better and does not care. However, a lukewarm individual knows much more and can do much more, but takes the easy way out. He is his own worst enemy because he knows he can be better, but does not want to be troubled. The cure is in God's Word in the Holy Bible. Repentance, confession, partaking in the Holy Eucharist, and all the sacraments make us spiritually rich. But you must ask for and pursue this wealth yourself. Be willing to repent and learn. Then, God will gladly bless you with more than what you ask for. You cannot afford to allow your love for Christ nor your fervor for living according to His righteousness to become lukewarm, mild, or temperate. Our Lord clarified His expectations of resisting spiritual lukewarmness saying;
"I know that you are neither cold nor hot. So then, because you are lukewarm, and neither cold nor hot, I will vomit you out of My mouth. Because you say, 'I am rich, have become wealthy, and have need of nothing'…and do not know that you are wretched, miserable, poor, blind, and naked…I counsel you to buy from Me gold refined in the fire, that you may be rich; and white garments, that you may be clothed, that the shame of your nakedness may not be revealed; and anoint your eyes with eye salve, that you may see. As many as I love, I rebuke and chasten. Therefore be zealous and repent" (Revelation 3:15-19).