Why is the main Affidavit in America the Bible?

Affidavits have always been a part of the law, courts and justice. However many people find it uncanny that in the United States, it is the Bible. The affidavit is mainly a written text which the witness swears on that what they say is the TRUTH. Why the Bible? Christianity may be the main religion in the United States but a fair amount of Americans are not Christian or would question just how realistic it …

…if the Bible is true, there are always people who will not believe in the Bible and its text, and who is a major witness to a crime. It may lead to the wrong men and women serving the wrong sentence. And so probably a majority of people will protest against this and say that this is not true. But are all Americans Chritians? Do all Americans believe in the Bible? I rest my case.