This liturgical season of Lent goes way back. There seems to have been an early impetus for the Church to prepare for and commemorate Jesus’ suffering, death, and Resurrection by means of fasting.
If you'd like to take a deep dive into the history, I recommend this article in the Roman Catholic Church’s official encyclopedia, the New Advent: https://www.newadvent.org/cathen/09152a.htm
You can follow links in this article to articles on “fast” and “mortification,” which make for fascinating reading (if a bit dry, as you might expect from an encyclopedia article).
I tend to approach these things from a Protestant perspective, and more narrowly from a Reformed perspective, which biases me toward wearing certain lenses. One lens is polarized for legalism in the Church. You will see in the above article a broad historical concern for what the rules of Lent and the rules of fasting dictate. When fasting, at what time of day can one eat, what materials can one consume, and how much of each? While we may no longer adhere to the rules of Roman Catholic doctrine, seeing these rules unfold historically with such firmness reveals that we tend to substitute attention to the discipline for attention to the effects.
If you dig deep into the article on fasting in New Advent, you find an assertion that the specifics of fasting are really important because fasting is a tool of expiation for sin. And I see this topic, too, through another lens.
We have a long tradition that views fasting as somehow dealing with the problem of past sins. This perspective sets off alarm bells for me, because it threatens to dilute the conviction that we are fully forgiven in Christ, and do not have to bear the guilt or shame of past sins. This perspective reminds me of how we have on occasion taught our children to do chores: as punishment. Your consequence for bad behavior is to vacuum the house. But wait–isn’t the purpose of vacuuming so that we and guests can better enjoy the comfort of our home?
These two issues, legalism and penitence–and we might add limiting Christian discipline to seasons–remind us that it's hard to keep our eyes on the prize. We have been given the Spirit of Christ, forgiven from past sins and empowered to grow in love all of our days.
I would rather set aside the questions of “Am I following the fasting rules just right?” and “Am I fasting enough to atone for my sins?” I would rather pick up the question, “What is God showing me about myself in this fast?” I would rather ask God, “How do you want me to grow into the image of Jesus?”
When we offer ourselves to God, accentuating that offer with the space opened by fasting, God fills that space with the Spirit. And the Spirit always comes to make us more like Jesus, empowering us with grace and peace.
~ emrys