You say that the "One Thing You Must Do" despite our busy schedules is prayer and Bible study. Yet aren't devotions, for many, just one more thing that already busy Christians feel they need to add to their plate?
They certainly can be. I want to be careful not to suggest that we must take X number of minutes in the Bible and prayer every day or we aren't Christians. At the same time, I think the Mary and Martha story shows up at the end of Luke 10 for a reason. After the disciples are sent out on their short term mission trip, and after the moving story of the Good Samaritan, it's no accident that we have this episode where Jesus reminds us not to neglect the one thing that is most needful. If we are busy in a hundred good things—even great things, gospel things, glorious things—but don't sit at the feet of Jesus, we are busy in the wrong ways.