It works if you believe your religion. If you are a roman swearing by Jupiter you are asking Jupiter to strike you with lightening if you lie - and as a believer you honestly think that will happen. since most people are christians they swear on the bible knowing they are asking to go to hell if they lie - even if otherwise they are good enough to be forgiven. Muslins generally swear of the quarn - to similar reasons (i’m not clear on what muslins believe happens if they lie, but they believe in it which matters)

of course a large number of people are not religious these days - so this is often pointless.