The two greatest threats to the church are the two greatest threats to your salvation. Satan destroys the church one person at a time. 1 Peter 5:8. He is unremitting in his attacks. He marshals forces and pressures to defeat the work of God by enticing the saints to defect.
The two greatest threats we face are iniquity and idolatry. Iniquity causes the people of the Lord to stumble in unbelief; idolatry is placing anything on the throne of your heart other than the Lord. God said through is prophet: "Son of man, these men have set up their idols in their heart, and put the stumbling block of their iniquity before their face..." Ezekiel 14:3.
Note that the idolatry was in their heart; and not a statue on the shelf. The light of the glorious gospel has shined in our hearts. We are the temple of the Holy Ghost. If we are not careful, the light of God can go out in our lives, and in our churches. See 1 Samuel 3:3. But like the priests of old, we are to see to it that the fire of God never goes out on the altar. Leviticus 6:13.
There are forces in this world that would extinguish the light. The true church of Jesus Christ does not belong to this world. Romans 12:2. It is a small outpost of Heaven, surrounded by hostile forces, deep in enemy territory. its only chance of survival is to be protected by "air cover" from Heaven - with regular supply drops. There is a war of epic proportions being fought between the forces of good and evil. Ephesians 6:12. There is animosity between God's church and the devil's world.
They cannot co-exist in harmony. Either the church will triumph and separate the saints of God from the world; or the world will win out and separate the saints from the church. The influence of the world separated Demas from the work of God. See 2 Timothy 4:10. Satan's greatest tools are iniquity and idolatry. Iniquity is a direct attack on the church. it may be subtle, but it challenges the teaching of the minister, the authority of the church, or the operation of the Spirit. God hates iniquity. Psalms 5:4-6.
But sadly, most workers of iniquity do not know that they are working iniquity. See Ezekiel 33:30-33. It is doubtful that Hymenaeus and Philetus knew they were workers of iniquity. Neither did Diotrephes. But they were. They spoke dreams and words - but they were not of God. Jeremiah 23:25-28. Idolatry is even more subtle than iniquity. It is an indirect attack. it is far more than worshipping an idol. it is anything that lessens your love and commitment to the Lord and His work.
There is much idolatry in the 21st century. Idolatry is not the mere bowing down to worship statues - rather, it is a frame of mind. Anything that diminishes your ability to love the Lord with all your heart, soul and might, and to love your neighbor as yourself, is idolatry. Matthew 27:37-39. Ezekiel 14:3 says that people had set up idols in their heart, not a statue in their living room.
Worldliness and materialism are idolatry. If the enticements of this world separate you from your first love, you have become an idolater. If we align with the worldly order and system, we commit spiritual adultery. God's people have a mission: to be a light to the world. Isaiah 49:6. To fulfill that mission there must be a separation from the world. Ezra 9:1-2, 12-14.
Ministers are to feed God's people with knowledge and understanding. Jeremiah 3:15. God sets watchmen to warn the saints of the Lord against iniquity and idolatry - lest they grow cold and draw back from the very thing that can prepare them to inherit eternal life.