Like ancient Israel, we can easily fall back into our former ways. The Israelites rejected the law of God and relied on the traditions of Gentile nations. Elijah had to take drastic measures to prevent Baal worship from completely eradicating the worship of the true God (I Kings 18:20-40). Some of Judah's kings spent years tearing down shrines and high places to foreign gods (II Chronicles 34:1-7).
Christ warned the Pharisees: "For laying aside the commandments of God, you hold the traditions of men." For example, Christmas and Easter are traditions of men, but they are lies. What happens if a person, trying to establish a religion, mixes falsehood with the truth of God? Recall God's wrath when Aaron made a golden calf at the urging of the Israelites in the wilderness and proclaimed a feast to the Lord (Exodus 32:1-5). Observing Christmas and Easter in the name of Christ is no different.
Blending the lies of this world with the truth of God produces a foul mixture called syncretism (James 3:10-13). "Christian" religions of this world have mixed the traditions of paganism with some of the truth of God's Word. This is no different from what Israel was doing when Amos wrote back in 760 BC. Since their rejection of the house of David under Jeroboam I, the Israelites had practiced a syncretistic religion (Amos 5:21-26; 8:14; I Kings 12:25-33).