The Covenant Fast

Psalms 50:5 (AMPC): Gather together to Me My saints [those who have found grace in My sight], those who have made a covenant with Me by sacrifice. 

If you search the scriptures, every act of worship was always done through some kind of sacrifice. A sacrifice is giving or offering something to God that is valuable to you to show your faith, trust, and love for the God that you are sacrificing unto. It is your worship. That is why when we fast, we are willing to give up something that cost us everything which is our Utmost for His Highest. 

Blessings