Heresey, by my definition, is something that will keep a person out of heaven.
I'm not a Calvinist, but I would not call a Calvinist a heretic just for that.
I've met people who have called me a heretic because I'm not a Calvinist.
Most of my Pastor friends are Calvinists, but that does not stop us from working together. Godly Calvinists can work with non Calvinists in perfect harmony.
To deny the Deity of Jesus, that is heresy.
To deny the Virgin Birth, or deny the resurrection of Jesus, that is heresy.
The question of Calvinism is a "difference of opinion".
Here is a statement of faith which shows the essentials of Christianity. Any thing that varies from this would be called Heresy.
Statement of Faith
• We believe the Bible to be the inspired, the only infallible, authoritative Word of God.
• We believe that there is one God, eternally existent in three persons: Father, Son and Holy Spirit.
• We believe in the deity of our Lord Jesus Christ, in His virgin birth, in His sinless life, in His miracles, in His vicarious and atoning death through His shed blood, in His bodily resurrection, in His ascension to the right hand of the Father, and in His personal return in power and glory.
• We believe that for the salvation of lost and sinful people, regeneration by the Holy Spirit is absolutely essential.
• We believe in the present ministry of the Holy Spirit by whose indwelling the Christian is enabled to live a godly life.
• We believe in the resurrection of both the saved and the lost; they that are saved unto the resurrection of life and they that are lost unto the resurrection of damnation.
• We believe in the spiritual unity of believers in our Lord Jesus Christ.
Of course the last line refers to real and genuine believers.
When you find some who aren't in spritual unity, there is a good chance that person isn't a real believer or possibly that person is just not a very mature Christian.
After all, none of us are perfect, we are all sinners.
If you have any more question, feel free to ask.
Pastor Art