I agree with you that to repent is needed, but let me clarify something.
You can not learn how to swim unless you get wet. Sometimes seems that we ask for the impossible.
How can anyone deny his (false) faith in order to receive a new (true) one if he can not taste the truth. It is not enough to believe by logic or by knowledge. The most important of all is to have the experience of truth.
Orthodox Church is a Church of empirical faith. If we deny the "empirical" part we may not ask for the faith.
The God of Calvinism does not exist. The God of Orthodoxy does not exist.
The God of Katherine, of Peter, of John, of Mary truly exists. The God is a Trinity relation of Three Persons. He does not exist according to theological systems or truths or dogmas.
He is the God of Abraham, Isaac, Moses, Jacob, George, John, Mary. He is the God of living persons.
You wonder whether you are talking about the same God and Christ when you talk to another human. You may surprisingly find that you are talking about exactly the same God. Live the life of others and then you may find something new about them. You may find that they know the real true God the same Who you know and they have not realize it. That they have a (false) faith that contradicts their true experience of God. Then you may reveal to them the Truth they already have tasted but they are unaware of.
This is how ancient Christians brought other people to Church. By living their lives and by revealing to them that they share the same experience regarding God. Then they show to them that their original faith was wrong and that the new Cristian faith was compatible with their original true experience of God.
katherine speak with the other person about his/hers God and your God, not about the God of Calvinism and the God of Orthodoxy. Live his/her life. I assure you it is not so different of yours.
Finaly if Church cut off Arius and others from His Body this is not the way to live you life with everybody.
Excommunication is trying to give a second chance to a person that previously experienced the life of Christ and now has stoped doing so. Excommunication is not a punishement but a proposal for "the lost lamb" to be found.
Excommunication is not isolation and loneliness.
Excommunication is a declaration to the believer of wrong faith that he needs to live the Life of Chirst.
Excommunication is not a fence that protects right believers from delusive ones.
Finaly I agree with you that Truth is not a relative thing. It is a personal experience.