Where is the command in Torah to fast on Yom Kippur? I thought it only said that we are to afflict our souls.
The command to commemorate Yom Kippur or the Day of Atonement appears several times in the Torah (Exodus 30:10; Leviticus 16:30; 23:27-28; Numbers 29:7-11), with each section of commandments giving specific details. One of those specific details—and arguably the most important—is listed in Leviticus 23:27: “it shall be a holy convocation for you, and you shall humble your souls.” What does the clause v’innitem et-nafshoteikhem mean, exactly? The verb anah, appearing the Piel stem (intensive action, active voice), is defined as “humble onesf., mortify onesf. (by fasting)” (CHALOT). So from a lexical standpoint, “afflict your souls” (KJV) or “self-denial” (NJPS) can definitely mean that one is to fast on Yom Kippur.
Surveying Jewish history, it is very easy to see that fasting—abstaining from food—was most definitely the traditional interpretation and application of what it means for one to afflict himself or herself on Yom Kippur by the time of Yeshua. A direct reference to Yom Kippur appears in Acts 27:9, which says that on Paul’s way to Rome “considerable time had passed and the voyage was now dangerous, since even the fast was already over, Paul began to admonish them.” Here “the fast” (Grk. tēn nēsteian) is undoubtedly Yom Kippur. Commentators are almost unanimously agreed that this is Yom Kippur. F.F. Bruce explains, “By the ‘Fast’ [Luke] means, of course, the Great Day of Atonement, which falls on Tishri 10.”
In the Jewish tradition, there are exemptions from fasting on Yom Kippur. Via the Rabbinic principle of Piku’ach Nefesh or Regard for Human Life, infants and the infirm and/or the elderly are permitted to eat on the Day of Atonement, but preferably with basic staples and water. However, if one can adequately fulfill the requirement to fast, one is expected to do so. The fast of Yom Kippur is to focus oneself entirely on God and in confessing any sin—individual or corporate—and to intercede for His mercy upon Israel and the world.
While many Messianics find it difficult to fast on this one day throughout the year, it is notable that many of the greats who have served the Lord over the centuries made it a regular practice. John Wesley often fasted between Thursday afternoon and the late afternoon or evening meal on Friday, every week. Many people who feel the need to focus themselves on the Lord in intense periods of prayer fast for weeks or a month at a time, although they will normally drink water. Messiah Yeshua Himself fasted for forty days in the wilderness after being immersed by John the Baptist (Matthew 4:2; Mark 1:13; Luke 4:2). So certainly, fasting for a single day on Yom Kippur should not be construed as any kind of burden, but a healthy faith practice that we probably do not do enough.
 CHALOT, 278.
 F.F. Bruce, New International Commentary on the New Testament: The Book of the Acts (Grand Rapids: Eerdmans, 1983), 506.