because you know that the testing of your faith develops perseverance.I have come to believe that perseverance is one of the key components in true Christianity. The one who sticks to their faith in Christ, and does not fall away, is the one who is exhibiting true faith. Faith that falls away isn't real faith.
Notice even in this verse that perseverance springs from a tested faith. It reminds me of a chemical reaction. To the eye hydrochloric acid and water look the same, but if you drop a cut penny down in the acid, it will have a reaction with the zinc in the penny. The penny will begin to bubble, producing hydrogen. If you drop a penny in water, nothing noticeable happens.
The same is true of faith. The person with faith, when the trial is added to their life, it produces perseverance. This why it is called the "...testing of your faith..." A similar passage to this one is I Peter 1:6-9,
In this you greatly rejoice, though now for a little while you may have had to suffer grief in all kinds of trials. These have come so that your faith—of greater worth than gold, which perishes even though refined by fire—may be proved genuine and may result in praise, glory and honor when Jesus Christ is revealed. Though you have not seen him, you love him; and even though you do not see him now, you believe in him and are filled with an inexpressible and glorious joy, for you are receiving the goal of your faith, the salvation of your souls.(You can read the commentary on these verses here.)
Trials will come, and if your faith is real, so will the perseverance.