You raise an interesting question: Can a Christian sin and even sometimes spout heresy without being a heretic? It appears that Scripture answers this query in the affirmative.

Did not Paul condemn Peter (and Barnabas) of hypocrisy and teaching by example a works-based salvation, which was a form of heresy? Yet they obviously were not heretics but Christians—and Peter was appointed as an apostle.

More; Can Christians Promote Heresy?