This is a new prayer I wrote recently for praying on fridays while fasting. It is about half scripture and half original wording of a totally unoriginal theology. This scripture is mostly from John 6.
Lord God help me to not labor for the food that perishes
But for the food that endures to eternal life
Which the Son of Man has given to me
My Father you have given me the true bread from heaven
For the bread of God is he who comes down from heaven
And gives life to the world
Lord Jesus Christ you are the bread of life
Whoever comes to you shall not hunger
And whoever believes in you shall never thirst
Holy Spirit feed me on this Friday with Spiritual Food
As I abstain from ordinary food strengthen me with the Crucifixion
For on its terrible violence Jesus Christ proclaimed it is finished
Help me to truly grieve my sin
And trust in this expiation and propitiation made by my High Priest
And that out of that trust
You would give us the grace
To go to Jesus outside the camp and bear the reproach he endured
For all your works Lord God I humbly thank you
One of the reasons Christians fast and abstain on Fridays is that in order to imitate Christ holistically we must imitate him physically. That’s the reason for the Church year. The Old Covenant feasts, seasons, etc. may still be permissible to observe but the church wanted to structure its life around the life of Christ. By making our year follow Christ we learn to follow Christ better ourselves. And each week also needs to reflect that it is part of an entire year of living for Christ. So on Friday we commemorate, mourn, and imitate his death by abstaining from Meat and Cheese and fasting from food in general. In the West Roman Christians have allowed Fish as a substitute because of the different words in Latin for meat, the primary one carno- referring to hot blooded animals (though some fish, in particular Sharks are hot blooded but the range of carno- excludes fish). In the East Greek Christians abstain from all meat assumedly for the same reason; the Greek word doesn't exclude fish. But whichever path you take you should be abstaining from rich foods, especially ones you find particularly pleasurable. The point is Self Denial. This was a big part of Reformation Piety though it was not generally worked out in this way, John Owen etc. considered these sorts of things Romish. Not drinking soda or alcohol would be another good way to observe Friday fast/abstinence, along with the more traditional means. Maybe even chocolate or candy. But the point is to deny yourself as Christ denied himself in order that you might put to death the flesh and gain mastery over your body just as Christ did for you.
So in the prayer itself we see two steps of theology. The first is meditation on scripture. We bring John 6 to our minds understanding that Christ is our true food and begin to prepare ourselves to receive the Lord's Supper on Sunday. Then we bring Christ's sacrifice and our dependence on the Holy Spirit before our minds. Understanding that our ability to mortify the flesh comes from God and nowhere else. But it’s more than just intellectual. Because this prayer is asking that we would have the strength to imitate Christ's total sacrifice by referencing Hebrews 13 at the very end. That our lives would become like His Life.
For a good Evangelical theology of fasting see Dallas Willard, particularly Spirit of the Disciplines.