The Bible never makes fasting a requirement for the New Testament believer. In fact, even in the Old Testament, the only mandatory fast was for the solemn Jewish festival of the Day of Atonement, which finds its fulfillment in Christ. Fasting is also never upheld as a sign or standard of Christian maturity, nor is there any clear biblical principle that would lead to the idea that all Christians everywhere are supposed to fast. We can, therefore, safely conclude that Christians do not have to fast and that no one should stand in judgment over a Christian brother who does not fast. With all of that said, however, the Bible does mention fasting in the early church, and Jesus did give His disciples instructions about fasting, so there is also good reason to say that fasting can be a godly Christian practice.
Throughout the Bible, fasting is associated with earnest prayer. It is practiced most often in the context of mourning, grieving, or repentance, but also before serious decisions or leading up to times of trial when one will especially require God's aid and favor. We see this, for example, in the book of Esther. Esther is going to risk her life by going unlawfully into the presence of the king to intercede for her people. Preparing for this, she says:
"Go, assemble all the Jews who are found in Susa, and fast for me; do not eat or drink for three days, night or day. I and my maidens also will fast in the same way. And thus I will go in to the king, which is not according to the law; and if I perish, I perish," (Esther 4:16).
Nehemiah is also said to have fasted for days before going to the king to appeal to him to rebuild Jerusalem's walls. In the midst of his fasting he prayed:
"O Lord, I beseech You, may Your ear be attentive to the prayer of Your servant and the prayer of Your servants who delight to revere Your name, and make Your servant successful today and grant him compassion before this man," (Nehemiah 1:11).
This theme of fasting in earnest prayer to seek God's favor over especially significant decisions or actions carries on into the New Testament. For example, when Paul and his missionary team were appointing elders in the churches they established, it says:
"When they had appointed elders for them in every church, having prayed with fasting, they commended them to the Lord in whom they had believed," (Acts 14:23).
Choosing pastoral leadership to shepherd the local church is not a small matter. It is a great weight of responsibility for the men chosen and has a tremendous impact on the spiritual life of the church as a whole. The early Christians did not take this lightly, and only after earnest prayer with fasting did they take such a significant action. They knew they needed God's guidance, blessing, and help in a profound and special way in such a moment as this, and they took time to cry out to God in a focused and sacrificial way by pairing their words with an act of faith and self-denial. That act was fasting.
This does not, of course, mean that any time a Christian makes a big decision without fasting that they are making the decision faithlessly or failing to trust in God. Fasting is not mandatory to be faithful, but it can be very helpful in being faithful if it is practiced in a biblical and Christ-honoring way. The early church knew this, and so fasting was a part of their prayer life, particularly in times of particular urgency like this.
Jesus also gave instructions for fasting as a regular part of our private, individual worship of God. Matthew 6:16-18 records Jesus teaching:
"When you fast, do not put on a gloomy face as the hypocrites do, for they neglect their appearance so that they will be noticed by men when they are fasting. Truly I say to you, they have their reward in full. But you, when you fast, anoint your head and wash your face so that your fasting will not be noticed by men, but by your Father who is in secret; and your Father who sees what is done in secret will reward you,"
Here we see fasting as a personal act of devotion. It is something we do secretly, just between God and us. If a man goes a day without food to honor God and never tells anyone else that he did so, what does he get out of it? Only the pleasure of His God, and that is the point. Secret fasting, along with secret prayer, at acts of faith that God is really there and really sees us doing these things for no other reason than to please Him. This pleases God and strengthens our own faith, and in this way is a positive and healthy thing for a believer to do if we do it in the way that Jesus prescribes. Jesus does not say, "you must fast." But He does tell us how we ought to do it when we do fast. Fasting can be a very helpful aspect of our individual walk with Christ and communion with the Father.
So we see that fasting is indeed a New Testament practice. It has both cooperate and individual expressions for different reasons and circumstances. While no one's salvation or sincerity of faith should ever be called into question over a lack of fasting, we should be careful that our reason for avoiding fasting is not merely our great abundance and our personal comfort and love of food. Not only is this self-centered, but also short sighted! Our relationship with God and our desire to be in His will should always come first, and for many of us, fasting may be a helpful tool in those areas.