What Catholics and other Christians observe during Lent is altogether in line with biblical teaching.
My specialty as an apologist is “biblical arguments for Catholicism.” I enjoy that aspect of my work a lot because the Bible is the great “common ground” that all Christians share (and I strive to be ecumenical). We all reverence Sacred Scripture and believe it is inspired revelation. Along these lines, someone asked me a few years ago on my Facebook page: “How come we celebrate Lent when the word is not in Scripture?”
Lent comes from the Old English word for “spring”. “Trinity” (that is, the word) is not in Scripture, either. Christian church buildings are not in the New Testament at all; it describes Christians as meeting in homes. “Original sin” (the term) is not in the Bible (whereas the concept is), nor is the word “theology”.
The biblical books are not listed in Sacred Scripture itself. The Bible never refers to evangelical “staples” like the “altar call” or “personal relationship with Jesus” or “have you accepted Jesus into your heart?” or many other ways of describing things in the Christian life that lots of people probably assume are “in there” somewhere (sort of like all the ingredients in a tasty vegetable soup).
But the practices and beliefs regarding Lent (as with these other topics) are, assuredly, eminently biblical. In other words, we can confidently assert (especially when challenged a bit) that “if we truly want to be biblical Christians and to fully conform our lives to scriptural guidelines and models, Lent (or something similar to it) will be part of our walk with Christ.”
Fasting and abstinence: central practices during Lent, are quite biblical activities. The evidence is massive: largely from the Old Testament, but also in the New. Here is just a small sampling:
Ezra 8:21 (RSV) Then I proclaimed a fast there, at the river Aha'va, that we might humble ourselves before our God, to seek from him a straight way . . . (cf. 8:23; 9:5)
Nehemiah 9:1 . . . the people of Israel were assembled with fasting and in sackcloth, and with earth upon their heads. [Ash Wednesday!]
Psalm 69:10 . . . I humbled my soul with fasting, . . .
Luke 2:37 . . . She did not depart from the temple, worshiping with fasting and prayer night and day.
Luke 7:33 For John the Baptist has come eating no bread and drinking no wine; and you say, “He has a demon.” (cf. Mt 11:18; Lk 1:15)
Acts 13:2-3 While they were worshiping the Lord and fasting, the Holy Spirit said, “Set apart for me Barnabas and Saul for the work to which I have called them.” Then after fasting and praying they laid their hands on them and sent them off.
Romans 14:3 Let not him who eats despise him who abstains, and let not him who abstains pass judgment on him who eats; for God has welcomed him.
Jesus assumed that fasting would be a regular part of the Christian's life, when He said, “And when you fast . . .” (Matthew 6:16; repeated in 6:17).
The forty (or so) days of Lenten observance have several parallels in Holy Scripture: Moses’ fasts on the holy mountain (Ex 24:18; 34:28: “And he was there with the LORD forty days and forty nights; he neither ate bread nor drank water”; Dt 9:9) and his intercession for Israel (Dt 9:25), Elijah’s journey to Mt. Horeb (1 Ki 19:8), Ezekiel’s lying on one side (Ezek 4:6), and Christ’s fast in the wilderness (Mt 4:2).
The premise underlying all these practices is mortification: the “subduing” of the body (or, in a similar sense, the “flesh” over against the spirit) for spiritual purposes. This is controversial among some, too, yet it has an explicit biblical basis as well, particularly in this passage from St. Paul:
1 Corinthians 9:27 but I pommel my body and subdue it, lest after preaching to others I myself should be disqualified.
A survey of this verse in different Bible versions is very interesting: NIV, Beck: “beat”; NEB: “bruise”; Williams: “beating and bruising”; Barclay: “batter”; NASB: “buffet”; NRSV: “punish”; NKJV: “discipline”; Goodspeed: “I beat and bruise my body and make it my slave”. Commentators dispute precisely what St. Paul means, and the Catholic position does not absolutely require only one interpretation of this passage.
But in any event, even if it's merely metaphorical, it fits into the overall scriptural theme of self-sacrifice, as outlined above; therefore it is expressing a principle or premise regarding some sort of asceticism or what we might call self-discipline; however one specifically interprets it.
Bodily mortification is further indicated in the Bible in a variety of ways. One common theme is that of “sackcloth”: which is basically a form of hair shirt. It appears often in the Old Testament. For example, the prophets Isaiah (Is 32:11) and Jeremiah (Jer 4:8; 6:26; 49:3) recommended it. It was not only used for mourning, but also for repentance (2 Ki 19:1-4; Neh 9:1-2; Jon 3:5-6), and to foster prayer and a deeper walk with God (Ps 35:13; Dan 9:3; 2 Mac 3:18-19). Both of these aspects are part of the practices and beliefs of Lent. The Day of Atonement (Yom Kippur) is similar to Lent also (Lev 16:29-34).
Sackcloth is not absent from the New Testament. St. John reports God saying, in a period near the end of the age: “I will grant my two witnesses power to prophesy for one thousand two hundred and sixty days, clothed in sackcloth” (Rev 11:3; cf. Mt 11:21).
Bottom line: what Catholics and other Christians observe during Lent is altogether in line with biblical teaching. Those Christians who don't “do” Lent (like myself in my first 32 years: though I would fast at times) miss out on these important penitential aspects of the faith and the spiritual blessings and graces obtained therein. What we find in inspired Scripture (especially if it is repeated many times) is important and is intended by God for our spiritual well-being.