This is Apple's "universal links" feature – the idea is to allow users to open content in the app when they're on a mobile device, because the app is better optimized for a smaller screen and touch navigation. You can change your preference for opening universal links in Safari or Gospel Library by long-pressing the LDS.org link, then choosing "Open" (to open in Safari) or "Open in Library" (to open in Gospel Library). iOS will remember your preference for future LDS.org links.
Samuel Bradshaw • If you desire to serve God, you are called to the work.