The word "heresy" means choosing or siding against orthodoxy - which is from the Greek "orthos" meaning "right", "true", "straight". So, I would say YES the denial of the Deity of Jesus Christ, that He is Almighty God incarnate, is heresy.
Can a person become a Christian not fully knowing or understanding this very basic tenet? Perhaps, but can he vehemently deny and reject it as the truth after it has been shown to be a Biblical and truthful doctrine and still claim to be a Christian? I find it impossible to believe a Holy Spirit indwelt believer could do so. In fact, it is the Holy Spirit that illuminates this truth to our hearts so that we accept it by faith even when we cannot FULLY comprehend how it all works with our mortal and finite brains.
So, I would say YES the denial of the Deity of Jesus Christ, that He is Almighty God incarnate, is heresy.
***Then it is proper to condemn such ‘christians’ as wolves in sheep’s clothing & pour harsh words on them just as Jesus did. They are false teachers, and spiritually dangerous ones at that.
Can a person become a Christian not fully knowing or understanding this very basic tenet? Perhaps, but can he vehemently deny and reject it as the truth after it has been shown to be a Biblical and truthful doctrine and still claim to be a Christian? I find it impossible to believe a Holy Spirit indwelt believer could do so. In fact, it is the Holy Spirit that illuminates this truth to our hearts so that we accept it by faith even when we cannot FULLY comprehend how it all works with our mortal and finite brains.
***At this point it makes sense to start sending truthful christians who want to know the historical evidence over to various sources.
For instance, I like the book “Jesus, God Ghost or Guru”
http://www.amazon.com/Jesus-Ghost-Christian-university-curriculum/dp/0310357616/ref=sr_1_1?ie=UTF8&qid=1388205819&sr=8-1&keywords=jesus+god+ghost+or+guru
And there’s the more recent book by Lee Stroebel “The Case for Christ”.
And, here on Free Republic, an entire book transcribed:
THE NEW TESTAMENT DOCUMENTS Are they Reliable? (Preface, Chapt. 1)
Sat 16 Feb 2008 02:24:23 PM PST · by blue-duncan · 7 replies · 103+ views
http://www.worldinvisible.com/library/ffbruce/ntdocrli/ntdocont.htm ^ | 1959 | F. F. Bruce
Various articles on historicity of Christ
http://www.freerepublic.com/tag/historicity/index?tab=articles
and so on... the apologetics section of the christian bookstores is another good place to start.