Question: "Why did Jesus speak so strongly against lukewarm faith?"
Answer: In Revelation 3:14-21, the Lord is describing the “lukewarm” heart attitude of those in the Laodicean church, an attitude manifested by their deeds. They were neither cold nor hot in relation to God, just lukewarm. Beverages are better either cold or hot. Similarly, the Lord would rather that people be either hot (spiritually on fire for Him) or cold (rejecting Him outright). The Laodiceans understood the analogy because their city drinking water came from a spring six miles to the south over an aqueduct, and it arrived disgustingly lukewarm. It was not hot like the nearby hot springs that people bathed in, nor was it refreshingly cold for drinking. It was lukewarm, good for nothing. In fact, it was nauseating, and that was the Lord’s response to the Laodiceans—they sickened Him.
The letter to the church at Laodicea is the harshest of the seven letters to the churches in Asia Minor. By His indictment against their “deeds,” Jesus makes it clear that this is a dead church. That is not to say that there were no true believers there, only that the church as a whole was spiritually uncommitted. Jesus frequently equates deeds, or works, with a person’s true spiritual state. He said, “By their fruit you will recognize them” and “Every good tree bears good fruit” (Matthew 7:16-17). Clearly, the deeds of the Laodiceans were not in keeping with true salvation because the deeds of the true believer will be “hot,” reflecting the spiritual passion of a life transformed. Such are easily recognizable by the world. The lukewarm deeds, however—those done without joy, without love and without the fire of the Spirit—do more harm to the watching world than the deeds done by those who are completely cold to the things of God. The lukewarm are those who claim to know God but live as though He doesn’t exist. They may go to church, but their religion is self-righteous complacency. They may claim to be Christians, but their hearts are unchanged, and their hypocrisy is sickening to God.
The Lord rebukes and disciplines the true believers in the church of Laodicea, commanding them to repent. He sees their lukewarm attitudes as “shameful nakedness” that needs to be clothed in the white garments of true righteousness. He urges them to be earnest, or zealous, and commit themselves totally to Him, for He will reject their lukewarmness by spitting them out of His mouth. But our Lord is gracious and long-suffering. He will always receive to Himself even the most lukewarm Christian who comes to Him in repentance.
The Laodiceans enjoyed material prosperity that led them to a false sense of security and independence. The expression "I am rich, and have become wealthy" is a literary device that inverts the natural sequence for emphasis. Here it stresses that the wealth attained came though self-exertion. Spiritually, they had great needs. This self-sufficient attitude is a constant danger when Christians live lives of ease and enjoy prosperity.