because you know that the testing of your faith produces perseverance.
OJBBecause you have da'as that the emunah you have, when it is tested, produces savlanut (patient endurance).
RVKnowing that the proof of your faith worketh patience.
TCENTbecause you know that the testing of your faith produces endurance.
T4TGod sometimes allows you to experience difficulties in order to test whether or not you will continue to trust him. Whenever that happens, you need to realize more and more that if you continue to trust him, you will be able to bravely/patiently endure difficulties.