Final answer:
Calvinists believed that altars, statues, and images in churches were considered idolatry.
Step-by-step explanation:
The answer to the question is Idolatry. Calvinists believed that altars, statues, and images in churches amounted to idolatry, which is the worship of idols or religious images as if they were gods themselves. This belief was influenced by the Protestant Reformation, which sparked a wave of iconoclasm where religious imagery was destroyed. Calvinists, like John Calvin, actively eliminated imagery from their churches and regarded religious images as idolatrous.