Mortgage applications require credit checks, to make sure you're a safe bet for the mortgage provider. Since your credit rating is affected by owning a credit card – positively if you've managed to pay the balance off every month, negatively if you've accumulated debt on it and/or missed payments – your credit card will most certainly affect the success of the application. For that reason, it's a good idea to check your credit score before you apply for either a card or a mortgage.