I take GREAT exception to your easily proven false assertion that the Triune nature of our Almighty God is incorrect or that there are no DIRECT Scripture texts that say Christ IS God in the flesh.
***Would you characterize such a denial of the deity of Christ to be a heresy?
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.