Well to start let us break this in two parts:
Why Divine Command Theory is False?
1. The first objection to this theory goes back to Socrates in Euthyphro’s Delimma
Is an act right because God says it’s so, or does God say it’s so because it’s right?
If God’s declaration simply makes an act moral then this moral system is totalitarian. If it is the other way around God is not sovereign in forming morality and we should give it up.
2. Historical moral code in the Scripture is insufficient and cruel.
If we assume that Scripture is the main source of morality then we are in a weak position given all the evil acts condoned in major religious books (Slavery, Misogyny and….)
3.The moral character of God is not clear. All the arguments for the existence of god even if we accept them, don’t prove anything about the moral character of god and for all we know he/she/it can be evil (See Evil God Challenge by Stephen Law or Anti-God Peter Milican)
Why laws must be secular?
If the institution of the government and religion are not isolated any challenge to the laws would be considered blasphemy and progression in thought and ethics will be suppressed ( Dark Ages, Iran). Moreover, to include one religion as the main source of law is discriminatory to other religions and some modern concepts like human rights do not exist in Scripture.
Secularism ensures freedom of religion and welcomes challenge to orthodox ethical norms
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