Swearing on the Bible in Court

Truth oath in British and Commonwealth Crown Courts:
I swear by Almighty God
That the evidence I give
Shall be the Truth
The Whole Truth
And Nothing but the Truth
So help me God

Once you’ve taken that oath you are legally obliged to be completely honest. If you’re caught out lying you can be charged with Perjury, Contempt-of-Court or even Perverting the Course of Justice and Conspiracy to Commit a crime! It’s a criminal or civil offence that can be punished with a fine or jail, depending on the severity of the circumstances.

I've often wondered why it is that if you swear on the Bible in court, and then go on to tell lies, you're charged with an offense by the State. Why? You’ve sworn an oath of truthfulness, yes, but you’ve sworn it to God, a supreme authority and creator, and God is no agent of any State nor its law that you’re being tried under. Why is perjury not just a religious sin? Especially in the countries that, unlike my own the UK, have a separation of Church and State. In those countries if you commit perjury you should receive no charge at all, but just a recommendation from the judge that you go to a priest for confession!