Christians and Muslims are forbidden by God from charging interest. However most Christians and Muslims just ignore this rule and have invented reasons why it is ok.
In true Islamic banking they handle loans with no interest as a type of pooled investment. Your money could be loaned out but in return you could take out a loan at zero interest. At least I think that's right. I'm not an Islamic banking expert or anything.
The reason it is not allowed is because it historically leads to a type of debt slavery.
Im not a muslim so I cannot speak about that but for being a christian, yes the bible does say we must not charge interest (this also includes those who are Jewish), however the bible only speaks about not charging interest to those of our faith, but to a stranger (which may go from an unbeliever, outsider, or someone of a different faith), we can, but those who are a "stranger" can simply lie saying they are Jewish, Christian (or possibly Muslim?) to simply lie (and possibly steal) from a lender. Some christians may also not follow the OT due to most believing Christ fulfill the old covenant, and bringing a new one, but this isnt a religion debate, so its simply up to one own belief in that matter.
Religion aside (to prevent idiots from coming to start a argument), it is usually up to the person themselves. I've seen some lend without interest, and sometimes seen people pay back the loan with a tip for assisting them, even when they didnt have too. Some also charge very low interest. Whatever works for the lender, but dont be to greedy like some by charging crazy interest fees.
