What is the most important thing you did yesterday, or will do today?
What is the most important thing you did last year, or will do this year?
What is the most important thing you have done, or will do, in your whole life?
The answer in every case is: "Jesus Christ died for me."
The most important thing you have ever done, or may ever do, is accepting this truth: "Jesus Christ died for me so that I might know the Father in the Holy Spirit and grow to love those alongside me along the way."
As Paul Tillich said, "Accept that you are accepted."
Christian faith essentially means learning to love the truth that you are loved beyond all measure. This is no more a sheer mental act than learning to ride a bicycle. Because humans are essentially embodied rational beings, we naturally attain supernatural "soulful" maturity by way of "bodily" piety. This is why the sacraments are as concrete and repetitive as they are. The sacraments are the performatively necessary acts of faith that literally train our bodies to respond more and more readily and radically to the most important thing we can ever "do", namely, "Jesus Christ died for me that I might live for Him."
Thus we learn to appropriate the most important thing in the world––our life, and the death of death, in Christ's death––by performing the seemingly most unimportant things in the world: moving our hands before us in the shape of a cross, kneeling as we enter a chapel, folding our hands as we pray, bowing our heads as we pass an icon or a statue of sanctity, running our eyes over persistently inscrutable (or numbingly platitutidinous) words in a bible, entering a small room and rehashing our most awkward moments, and so on.