Fasting was ordained by Christ himself; Jesus said “when you fast,” not “if you fast.” Clearly Christians ought to fast. Though their fasting observances and traditions were extra-biblical, the early Christians had good reasons for fasting or not fasting on specific days
(such as THE DIDACHE, CLEMENT OF ALEXANDRIA, TERTULLIAN, VICTORINUS, and PETER OF ALEXANDRIA).
Nevertheless, there is evidence from TERTULLIAN that such appointed fasting was not obligatory. Fasting on certain days was known as “stations.” These fasts were very early fixed for Wednesday and Friday (THE DIDACHE, CLEMENT OF ALEXANDRIA, VICTORINUS).
Monday and Thursday were the common Jewish fasting days and were considered the fasts of hypocrites in THE DIDACHE.
According to SECOND CLEMENT, fasting is better than prayer.
THE DIDACHE and JUSTIN MARTYR mention the practice of fasting for baptism.
Let us persevere in fasting (POLYCARP).
Fast and you will obtain from the Lord what you beg (HERMAS).
The soul, when but ill-provided with food and drink, becomes better (MATHETES).
Christians often fast two or three days in order to supply to the needy their lack of food (ARISTIDES).
JUSTIN MARTYR, CLEMENT OF ALEXANDRIA, TERTULLIAN, and HERMAS exhort Christians to keep the true fast of God, as Isaiah says
(Isaiah 58:1-12).
HERMAS and CLEMENT OF ALEXANDRIA also understood that fasting signifies abstinence from all evils whatsoever, in action, in word, and in thought itself.
The dead have been raised when the Church has entreated the prayer request with much fasting (IRENAEUS).