I have a friend who asked about fasting. All I could quote was Matthew 6:16 and Matthew 15:17-20. Can you tell me where it says that all food is good for you? I can't remember the passage. Also, could you explain Matthew 17:21? Does it mean you should fast? Is it something we should do?
There is a passage that states all meats are now acceptable as food: "For every creature of God is good, and nothing is to be refused if it is received with thanksgiving; for it is sanctified by the word of God and prayer" (I Timothy 4:4-5). Perhaps that is the passage you were trying to recall. A similar passage is found in Romans 14:20.
Fasting is something mentioned quite often in the Old Testament. It was used during times of deep grief, such as while mourning the death of someone (I Samuel 31:11-13; II Samuel 1:11-12) or when people learned that God was about to overthrow them (Jonah 3:5-9). It is understandable; when your stomach is in knots because of grief, who would want to think about eating. Fasting was also used to demonstrate humility. David used fasting to humble himself while praying for his enemies (Psalms 35:12-14).
But most importantly, fasting was used when a believer wanted to focus completely on God. It expressed an urgency to the prayers that food wasn't as important as an answer to one's prayers. Thus when an army was gathering to attack Judah, the king declared that everyone should fast while they sought God's aid (II Chronicles 20:1-4). When Daniel wanted to focus his attention on God, he fasted (Daniel 9:3). This is what Jesus was referring to in Matthew 17:19-21. Back in the days when demons were in the land, a demon could not be expected to leave by a casual rebuke on the part of a disciple. It would take an extreme focus on God and strong faith.
Warnings are also given that fasting can be misused. Fasting doesn't make God listen nor does it make the person who is fasting more righteous (Isaiah 58:2-4; Zechariah 7:5-6). Even in the New Testament, Jesus warned that fasting should not be used to be seen by other people (Matthew 6:16-18). It is a personal matter between the individual and God.
Zechariah prophesied that there would come a time when the need for fasting would end, at least for a while. "Thus says the LORD of hosts: 'The fast of the fourth month, the fast of the fifth, the fast of the seventh, and the fast of the tenth, shall be joy and gladness and cheerful feasts for the house of Judah. Therefore love truth and peace" (Zechariah 8:19). This was fulfilled while Jesus was here on earth among men. "Then the disciples of John came to Him, saying, "Why do we and the Pharisees fast often, but Your disciples do not fast?" And Jesus said to them, "Can the friends of the bridegroom mourn as long as the bridegroom is with them? But the days will come when the bridegroom will be taken away from them, and then they will fast" (Matthew 9:14-15).
Fasting continued to be used in the New Testament. The disciples fasted before major events and before major decisions (Acts 13:2-3; 14:23). It is not commanded that it must be done, such as saying that all Christians must fast on a particular date or for a particular occasion. However, it is still appropriate to fast, especially during times of deep grief or when you need God foremost in your mind as you make a major decision.