Isaiah 58:3

They could not understand why God had not blessed them-because they had fasted and humbled themselves (cf. Malachi 3:13-15). The only fast that the Mosaic Law commanded was on the Day of Atonement (Leviticus 23:26-32). But the Israelites also fasted voluntarily, even in Isaiah’s day. [Note: On the practice of fasting, see Kent D. Berghuis, "A Biblical Perspective on Fasting," Bibliotheca Sacra 158:629 (January-March 2001):86-103.] The reason God had not blessed them was that when they fasted, they did not demonstrate the attitudes and activities that fasting represented. They did not really sacrifice their own desires, and they continued to treat other people inconsiderately. They pursued their personal interests and worked when they fasted, and they insisted that their employees keep on working. They were practicing religious ritual to try to manipulate God into blessing them.

"But repentance is not for the purpose of getting God to do anything; it is an expression of the conviction that my ways are wrong and God’s ways are right, whether he does anything for me or not." [Note: Oswalt, The Book . . . 40-66, p. 497.]