Many words in English are based on the same root words, and knowing what these mean can help us with spellings.
The root 'cred' is found in a lot of English words. It comes from a Latin word meaning 'belief' or 'trust'.
If you are incredulous about something it means you find it hard to believe.
If you buy something on credit you don't have to pay for it at the time because the seller trusts that you will pay at a later date.