For Christians perjury is the sin of swearing on the Bible to tell the truth and then lying.  If this is a sin then what is it if they swear on a scripture they do not believe in?  Its legally perjury but is it sin?   

Most people these days lie under oath and they get away with it because it is very hard to prove they were deliberately lying. In the past it was believed that if you lied under oath you would be consigned by God to eternal damnation because you were calling the God of truth to bear witness to a lie. That was the rationale for swearing on the Bible. It was very useful for those who secretly sneered at the Church - and every Church contains a large number of people who do that – to give their lies more weight.
Few people know the Bible well enough to be able to decide if it is God's word or not and to swear on a book that you think contains fabrications or may do so is in itself perjury. You are basing your oath on a tissue of lies.
One thing is for sure, the person who swears outside of a court of law is definitely lying for they are asking for more faith to be put in their statements than normally would be. Their oaths mean nothing because the solemnity and the penalty of the law does not hang over them so why are they wasting their breath swearing if they are telling the truth? Yet we have Jesus and Paul swearing in the Bible outside of the courts. Jesus said that God bears witness to him which is the same in all respects as an oath. If God is truth then to tell any lie is calling on God to witness to a lie at least implicitly. You lie to destroy truth and you are hoping that God will not expose you and thereby imply that he agrees with what you are doing.