With normal use of the holster, No, the magnets will not damage your credit cards.
 
 

However, they are powerful magnets.  They certainly will damage your credit card if you bring them extremely close to the magnet.

 

Typically normal use has multiple layers before reaching your credit card even if you keep it in the pocket where you holster your firearm.  The layer order would be Magnet > Leather > Firearm > Leather > Pocket > Wallet > Credit Card.  We've done extensive testing and found that there is no issue keeping your wallet in the pocket near your holster.