‘Apostasy’ is not a word we use much these days, even less ‘heresy’.  At Grace they use the word ‘apostasy’ which is someone who has abandoned the faith, and ‘apostate false teacher’ someone who not only leaves but goes on to teach false doctrine.  These ideas are present in scripture. 1 John 2:19 is the idea of apostasy.  But I think historically the church has used a better word for those who actively teach false doctrine. Christ warned us of them in Matthew 7:15  They often do not believe they have left the faith, and even fight for their beliefs.  They are those who start denominations or cults. They are the Heretics.   Which I believe is the most accurate contemporary term for what Peter is talking about in II Peter 2  He even opens the chapter with the greek word that has become the basis for our word heresy.  In the Greek it was “a taking or choosing”.

The church has battled many such persons throughout the centuries.  In the USA we have the common ones the Mormons, Christian Scientist, Seventh-day Adventists and Jehovah’s Witnesses.  But if there is a battle today, between the church and these heresies it is a cold war.  There is, however, a heresy that is alive and well, and the battle is as hot as ever.

Born in the year 570, we don’t often think of his followers as heretics.  He was certainly a false prophet of the type our Lord warned us about in Matthew.  In fact his followers call him an-Nâbî, “the Prophet”.  He is Muhammad.  It may sound strange to many to refer to Islam as a Christian heresy but that really is the best explanation.

First of all what is your definition of heresy?  If you compared the doctrine of the groups above or even historical heresies it is in full agreement in 90 percent of doctrine.  For the most part these groups agree.  But that last 10 percent is crucial. Heresy is that which is almost right but not quite.  Theologians like to point out that the first great heresy came down to a single letter, homoiousios (alike in substance) vs homoousios (the same substance).  But that one letter makes a lot of theological difference.  To the modern non-militant Christian this seems silly, this is what has led to the modern day movement to unite all the sects, agree on what we can agree on and live in peace and milky apathetic harmony.  Well they have simply forgotten their history, men fought and died to maintain orthodoxy for hundreds of years, shall we consider their lives meaningless?  I think not.

Second, Muhammed is the poster child for what Peter is talking about in the second chapter of his second epistle. v. 2 many will follow…because of them the way of truth will be reviled v. 14 They have eyes full of adultery, insatiable for sin,  v.18 they entice with licentious passions of the flesh, v. 19 they promise freedom but they themselves are slaves of corruption.  But the connection is not just in general.  Muhammed studied Christianity, he appreciated it’s structure and marveled at the way it spread so fast and so strongly throughout the world. He wanted this for himself, much like Simon the Sorcerer, he wanted to adapt Christianity for his use.  And he did.  Muhammedans beleive in Jesus and his teaching, but they believe that the bible was corrupted and that Muhammed’s writings are necessary to clarify and fix those corruptions.  You might not think there is much in common between Christianity and Islam.  But there are many paths by which can strike up a conversation and lead to the true gospel.

Further Reading: The Great Heresies, Hilaire Belloc  http://www.cs.cmu.edu/~spok/metabook/heresies.html
Icon from the Mégalo Metéoron Monastery in Greece, representing the First Ecumenical Council of Nikea 325 A.D., with the condemned Arius in the bottom of the icon.