John Gill emphasizes the duality of joy and sorrow in the Christian life as expressed in 1 Peter 1:6. He explains that believers rejoice greatly in their salvation, the mercy of God, and the hope of eternal life, despite experiencing heaviness due to various trials and temptations. These trials serve a purpose, testing their faith and character, and are temporary compared to the eternal glory that awaits them. Gill reassures that while afflictions may be necessary, they are not meant to cause despair but to refine and strengthen the believer's faith. Ultimately, the joy of salvation outweighs the temporary sorrows of this world.