"Now when Athaliah the mother of Ahaziah saw that her son was dead, she arose and destroyed all the royal heirs of the house of Judah. But Jehoshabeath, the daughter of the king, took Joash the son of Ahaziah, and stole him away from among the king's sons who were being murdered, and... hid him from Athaliah so that she did not kill him. And he was hidden with them in the house of God for six years, while Athaliah reigned over the land" (2 Chron. 22:10-12).
Athaliah was the daughter of Ahab, the wicked king of Israel. She married into the royal family of Judah and exerted her evil influence upon that nation. As the above text indicates, when her son was killed, she killed all her grandsons (his sons, the royal heirs to the throne) and reigned in his place. When the king's daughter saw what her grandmother was doing, she hid Joash to save his life.
Now, where would be a good place to hide something from this wicked woman? What place would she be least likely to stumble across something? The House of God.
If someone were wanting to hide something from you, would the church building be a good hiding place? Think about it.