Have you ever paid a bill? If you are an adult, and actually live (mostly) in reality then you have certainly paid a bill. In the past bills came in the mail. You would open the bill from you utility, credit card, mortgage, or gasoline company and at the top of the bill is a notice that says 'Due By Date'. If you don't pay the bill by that date your are in 'arrears', and your bill is subject to a penalty. In the case of a mortgage or utility if you don't pay after a specific amount of time (30, 60, 90 days) then you become subject to disconnect or foreclosure. That 'due by' notice is also known by the term Pay On Demand. This means that you have used goods or services ahead of the payment for same, and the entity that supplied the goods or services now, as in immediately, wants their money.
When you go to a 'brick and mortar' store this POD happens when you go to the cashier and enter a transaction in which you present what you have chosen from their stock and present a 'bill' (currency) in exchange for that item. When the transaction is completed to the satisfaction of both parties you can take the item with you out the door. If the transaction is not completed, as with a declined credit card, you must leave the item on the counter and go your way. If you decide to take the item anyway (as happens on a regular basis where I work) then you become a thief, and are subject to legal action.
Salvation works the same way. You have been sent a Pay On Demand notice by the God of Creation that says that you must pay for the sin of breaking of the Laws of God, and that the only acceptable payment is your eternal death. To live, you can have this penatly paid by the King of the Kingdom of Heaven IF you become a member of that Kingdom by "keeping the commandments of God, and holding to the words of Jesus" (Rev. 12:17).