This is historically inaccurate. According to the
Architect of the House's website, Teddy Roosevelt did NOT swear on the bible when he took his oath of office after President McKinley's death in 1903 - he merely raised his hand.
And nothing in the Constitution requires a Congressman to swear on the Bible. In fact, all Article VI requires is that the Congressman take an oath OR affirmation to support the Constitution. An AFFIRMATION doesn't require swearing on the bible at all, and gives an out for those with religious objections. Although I haven't done research yet, I'd be shocked if some Congressmen haven't gone the affirmation route in the past and not taken an oath on the Bible.
Its tradition one takes an oath on The Bible. It means you're swearing before God to the truth of your pledge to uphold your office.
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." -Manuel II Paleologus