heretics, apostates, and cultists

Error 
Everyone falls in this category on some point.  We see through a glass darkly and are in the constant process of learning and making course corrections as we mature spiritually.  The key is that we remain teachable and ready to change our minds, beliefs and opinions to conform to scripture under the guidance and discernment of the Holy Spirit.  We must be ready to correct or abandon the teaching of any tradition, personality, doctrine, or "revelation" that contradicts the full counsel of God's word.


Heresy
When error becomes doctrine.  Have unscriptural views on issues that may not directly interfere with salvation.  Considers those who don't agree with their beliefs as being heretics.  While they may sow doubt about the inerrancy of scripture and take scripture out of context, they still refer to scripture as the source of their beliefs even though they bend it to conform to their pet doctrines and traditions.


Apostasy
Heresy gone to seed.  Heresy to the point that they have departed from the faith to follow traditions of men and doctrines of demons.  Their apostate view is the basis of their salvation.  While they may refer to scriptures taken out of context, they look to other sources as being equal authority to or higher authority than the Bible.  These sources may include "new revelation", "spiritual experiences", traditions, commentaries, ancient wisdom, additional books, authority of an individual (prophet or apostle) or council of elders.  Apostates will also change the definitions of commonly used terms so that they sound "mainstream" in their public statements, documents and marketing materials, but then begin to incrementally introduce their true meanings to recruits over time.


Cult
An apostate group that is typically exclusive (they alone know the secret and everyone else is wrong or corrupted), is typically centered around a particular personality who is the ultimate authority, and they typically do not introduce their more controversial doctrines until after a recruit is first initiated into the group.  Often will seek to isolate members from those friends and family outside the group.