I did a search of The Highway but I could not find a definition of a heretic. Let me, therefore, suggest this one from "Loci Theologica" by Johann Gerhard,
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For one to be properly called a heretic, it is required (1) that he be a person received by the Sacrament of Baptism into the visible church; (2) that he err in faith …; (3) that the error conflict directly with the very foundation of faith; (4) that to the error be added malice and obstinacy, in which he stubbornly defends his error, though repeatedly admonished; (5) that he stir up dissensions and scandals in the church and rend its unity.
The first requirement would eliminate members of non-trinitarian churches since they do not receive a valid baptism. The second and third requirements would eliminate those whose error does not conflict with the foundation (i.e., justification by faith alone in Christ alone). The fourth requirement would eliminate those who have not been repeatedly admonished. The fifth requirement would eliminate those who hold to heretical error but do not attempt to spread the error.
The most likely candidate for the title "heretic" would be a leader within a trinitarian church-body who has been repeatedly admonished concerning his fundamental errors by other members of his denomination. Despite this, he continues to spread his poisonous opinions.
