John Knox gave much attention to the commandments to pray.
Luther focused on the Third Commandment (Second Commandment according to the Lutheran division): "You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes his name in vain" (Exodus 20:7). According to Luther's understanding of this commandment, not taking the name of Jehovah in vain is a negative way of commanding that we are to use it only according to His word. Lack of prayer means not taking His name as He desires, and thus is a violation of this commandment.
On the other hand, Knox developed his position from a wider range of texts: Psalm 50:14-15, Matthew 7:7-11, Matthew 26:41, I Thessalonians 5:17, and I Timothy 2:13, 8. In his "Treatise on Prayer," Knox explained, "He who, when necessity constrains, desires not support and help of God, does provoke His wrath no less than such as make false gods or openly deny God." Thus, in Knox's mind, lack of prayer is tantamount to paganism or atheism!
Both men said - and I want to emphasize - that they did not mean the Christian of frail conscience who struggles to overcome his sense of unworthiness when approaching the throne of grace. We are unworthy! Anyone who approaches God on the basis of his own worthiness doesn't understand his sinful state nor the necessity of the atonement in Christ. However, for the believing sinner, that atonement covers his unrighteousness, so that he can come before a loving Father God. Consider the text that I would add to the list above, Hebrews 10. Consider especially Hebrews 10:14 and Hebrews 10:19-23.