Playing sports and gaining a testimony have many parallels. Such as hard work, enjoyment, and growth.
If you want to be good at any sport it's going to take some hard work. You can have talent but to really be something in the world of sports you have to devote a large amount of time to practicing. To be the best you can be it takes hard work. The same is true in the gospel of Jesus Christ. It often takes time in practicing correct principles such as: faith, repentance, and enduring to the end before you can become the best you can be. We have both heavenly and earthly coaches who will help us every step of the way.
Playing basketball for me has always brought much enjoyment. I love other sports too, but basketball has always been my favorite. I love the feeling of lacing up my shoes and stepping onto the court. Shooting around in my front yard was therapeutic for me. Don't laugh, it's true. It was an outlet that made me really happy and still does. Guess what, the gospel of Jesus Christ is the same way. We can find joy in His teachings and comfort in the scriptures. The Book of Mormon is also a great help in my life. It makes me feel better when I read its pages.
Most of all, the exercise you get from playing sports helps your body grow physically. Just as running strengthens your legs, reading the Book of Mormon strengthens your testimony and helps you grow spiritrually.