In the Book of Revelation, we see the Lord has some rather severe words about the lukewarm: "I know your works; I know that you are neither cold nor hot. I wish you were either cold or hot. So, because you are lukewarm, neither hot nor cold, I will spit you out of My mouth"(Revelation 3:16). Pope St. Pius V went so far as to say, "All the evil in the world is due to lukewarm Catholics." Pope St. Pius X showed no less disdain with, "All the strength of Satan's reign is due to the easygoing weakness of Catholics." Wow! All the evil in the world? All the strength of Satan's reign? Spit you out of My mouth? The level of fury toward the lukewarm is alarming. Why?
Here's why. Let's look at the passage from Revelation. Our Lord, first, wishes we were "hot or cold." It's easy to see why the Lord would wish someone were hot. Someone who is "hot" would be that soul blazing with the "fire of love" for God, with a fiery passion to do His will.
So, why would our Lord wish someone were "cold"? Someone who is cold has completely disconnected from God and can often even be antagonistic toward God. So, why would our Lord wish for that? Because "cold souls," who have outwardly drawn the line in the sand and proclaimed their stance in relation to God, have a greater chance for hitting rock bottom and giving their life over to God; they have a greater chance for conversion. Furthermore, a soul clearly disconnected from God does little to influence souls that are "hot" from straying away from our Lord, as "cold souls" are seen clearly for who they are and where they stand.
So, why the outrage toward the lukewarm? Because the lukewarm do the most damage. They are highly effective in modeling for others how to be a very poor or even a "fake" Catholic. While "cold souls" openly disavow any claim to be a faithful Catholic, lukewarm souls make that claim while they betray the Lord at every turn.
Lukewarm souls are those who have allowed their faith to diminish to little more than what appears to be faith in the eyes of man. Lukewarm souls will often "use" their Catholic faith to build their own personal brand. So, you'll often see politicians make the claim of being a devout Catholic while they advocate for a plethora of evil policies that horrify God. Or there are those lukewarm Catholics who see no problem voting these evil politicians into power. This is a-okay for the lukewarm, as they have convinced themselves that God's mercy extends to allowing every kind of sin. Which is why lukewarm souls may go to a communal penance service once or twice a year "to be seen," but really don't see any reason for this sacrament.
The most horrifying thing about lukewarm souls is that they usually possess the sins against the Holy Spirit that are referred to as "unforgiveable."
The unforgiveness is about unrepentance. Lukewarm souls have convinced themselves, and poorly modeled for others, that "they don't need to prove their love for God." In other words, they think they can commit any sin and avoid prayer and any charitable work beyond basic obligations, and their ticket is punched because of "mercy." And, of course, nobody should "judge" anyone, according to the lukewarm soul. St. Thomas Aquinas said, "mercy without justice is the mother of dissolution; justice without mercy is cruelty. To correct the sinner is a work of mercy."
Lukewarm souls are like a dangerous virus that spreads throughout the Church. On issues of human life, sex, marriage, family, faith, and morality, many within the Catholic Church have absolutely no problem advocating for laws, policies, and politicians that clearly oppose the will of God. This "lukewarm virus" has spread so far that as many as 40% of Catholics favor abortion and 61% of Catholics support gay marriage. Now, you can see why our Lord, Pope St. Pius V, and Pope St. Pius X are so outraged by the lukewarm.
To combat the demonic viral spread of lukewarmness, we must cultivate in ourselves the Holy Spirit gift of fear of the Lord, also known as awe and wonder. Fear of the Lord is the gateway gift to all the gifts of the Holy Spirit. If your local parish is cultivating a watered-down, lukewarm version of Catholicism that avoids Church teaching on any "hot button" issues, "GET OUT!" Or, if possible, work with the pastor to cultivate reverent liturgies, frequent confession, ample opportunities for Adoration of the Blessed Sacrament, and the practice of many devotions. And, for heaven's sake, have your pastor's back when he hits hot-button issues. This all assists the faithful in "drawing nearer unto the Lord." This gives the soul immunity to the virus of lukewarmness.