There are many promises associated with fasting. I think it's interesting to look at the etymology of the word fast. It comes from a similar-sounding Old English word. This Old English word also formed the roots of other words that we still use today. For example, steadfast and fasten. By looking at how these other words are used in the scriptures we can learn more about promises that come to us through faithfulness and fasting.
Lehi spoke unto his son Lemuel saying, "O that thou mightest be like unto this valley, firm and steadfast, and immovable in keeping the commandments of the Lord!" (1 Ne. 2:10). Lemuel was urged to be steadfast - to be firmly rooted in the gospel and to never waver in keeping the commandments of the Lord. Elsewhere, the prophets have commanded people to be "steadfast and immovable, always abounding in good works, that Christ, the Lord God Omnipotent, may seal you his" (Mos. 5:15). When we fast, we subject the desires of the flesh unto the needs of the spirit. When we fast we follow God's commandment; we are proving ourselves steadfast and immovable and as such, through the Atonement of Christ, will be sealed to our Heavenly Father. We will be sealed for time and all eternity to our Eternal Father. His name will be fastened upon us.
To fasten something is to bind together, such as fastening two pieces of wood together with screws or ropes. Those things that are bound together become one. However, over time if care is not taken or if the fastener is weak, those that were fastened together may slip apart and weaken. Captain Moroni, that great champion for freedom, made a banner out of his coat and "fastened it upon the end of a pole" (Alma 46:12) that he might be able to travel from city to city, waving the flag of freedom, rallying the believers of God to fight for their God, their families, and their homes. He wanted them to hold fast to the words of the prophets and to their God. As we respond to Captain Moroni's call today and hold fast to our God, our families, and our homes, we will be sealed together with our Father.
When we fast, we sacrifice so that we might be able to become steadfast in our faith, becoming one with our God, to be fastened together with Him and Jesus Christ.
I close with the scripture that is the theme of this blog: "Organize yourselves; prepare every needful thing; and establish a house, even a house of prayer, a house of fasting, a house of faith, a house of learning, a house of glory, a house of order, a house of God" (D&C 88:119). As we establish our homes on the foundation of prayer, faith, and fasting, we will continue to grow in unity with the Savior and our Eternal Father.