Why was God so particular about the ark? (1 Samuel 6:19) -- "But God struck down some of the inhabitants of Beth Shemesh, putting seventy of them to death because they looked into the ark of the Lord. The people mourned because of the heavy blow the Lord had dealt them"
1 Samuel 6:19
ESV - 19 And he struck some of the men of Beth-shemesh, because they looked upon the ark of the Lord. He struck seventy men of them, and the people mourned because the Lord had struck the people with a great blow.
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I humbly think without going into so many reasons why our heavenly Father rightfully thought so, was it was God's sacred earthly womb (since He had decided to bring His glory down to us), and any sight of the priestly "womb'' other than those chosen to care or be in God's presence would be considered adulterous and thus punished for not being worthy and be able to tell anyone about what they saw. For lack of a better example, as if one saw my wife naked, she'd be embarrassed and wouldn't want anyone starting to gossip and spread rumors about what they had seen...I certainly wouldn't be pleased.