The devil hates the Word of God, like he hates innocence

Why does the NIV (and other versions of the Bible based on the corrupt Alexandrian manuscripts) just excise verse 21 from Matthew chapter 17? Because it references directly central practices taught in the Word of God, namely: prayer and fasting. This is the most important thing for the devil to excise from the Word of God because it is practical efforts based on a foundation of regeneration that separates the serious Christians from the Christians who are merely 'playing church' or just screwing around.

The word 'fasting' is also excised in Mark 9:29 by the versions based on the same corrupt Alexandrian manuscripts.

Just as the two great commandments of Jesus have deep and practical application prayer and fasting are the same. In fact, they correspond in foundational ways to the two great commandments to love God and to love your neighbor as yourself.

Just as the Bible presents the act of prayer as having large and diverse meaning in practice the same is true for fasting. Fasting is a primary act with which physically, emotionally, intellectually, and spiritually a Christian is separated from the world, the flesh, and the devil. The devil wants you neither knowing about nor practicing either. What better way to effect that goal than to just erase the words from the Word of God altogether, justifying the act by the sudden 'finding' of 'better manuscripts' in the era of the birth of the atheist 'higher critics'?

Here's another correspondence to think about: Protestants who have it shown to them how the devil has defiled and raped the precious Word of God and who just take it are similar to Roman Catholics who know their priests molest and rape their children...and just take it.

[For a list of doctrine changed by Bible versions based on the corrupt manuscripts - Bible versions such as the NIV, ESV, NASB, NLT, ASV - see here.]