That church definitely isn't Protestantism as a whole because there are many faiths within Protestantism, many of which contradict each other.
According to the written record of the Apostles anybody who beleives in Christ Jesus crucified and ressurected from the dead for the forgivness of sins and proclaims Him as Lord is a part of the church.
Seems clear God wants you to remain united to the body of believers and not divide it into hundreds of denominations.
The meaning of the word Greek word translated "heretic" (in Titus) is a divisive one, one who divides the body. Of course the beginning of divisiveness is in a lack of humility, gentleness, patience and not bearing with one another in Love.
When a group from one location tries to tell another group from another location that they should submit to the leadership of the first group because the first group were supposedly founded by somebody who is pretty irrelevent anyway (One of you says, “I follow Paul.” Another says, “I follow Apollos.” Another says, “I follow Peter.” And still another says, “I follow Christ.” Does Christ take sides? Did Paul die on the cross for you? Were you baptized in the name of Paul?), this is a failure of humility, it is in itself divisive (heretical).
The Roman church thus became the first denomination of an otherwise truly catholic body of beleivers.
When the Roman church declares that all of those who disagree are not part of the body and attempt enforcement is neither humble, nor gentle, nor loving.
It also completely misses the point of Christs teaching and example in that:
Who, being in very nature God,
did not consider equality with God something to be used to his own advantage;
rather, he made himself nothing
by taking the very nature of a servant,
being made in human likeness.
And being found in appearance as a man,
he humbled himself
by becoming obedient to death—
even death on a cross!
...and who taught:
An argument started among the disciples as to which of them would be the greatest. Jesus, knowing their thoughts, took a little child and had him stand beside him. Then he said to them, “Whoever welcomes this little child in my name welcomes me; and whoever welcomes me welcomes the one who sent me. For it is the one who is least among you all who is the greatest.”
The early Christians had one faith (as scripture teaches) that was transmitted from Jesus though his apostles (both orally and in writing according to scripture). Those who refuse to believe that faith are not following Christ as Jesus said "he who hears you hears Me and he who rejects you rejects Me and the one who sent Me. (Lk 10:16)"
Amen to that.
Disagreeing with even one doctrine of the Christian faith is a rejection of all of it because it means the person has no faith in Christ who revealed it. Such a person does not have faith because he is merely choosing to believe whatever feels right to him instead of putting his trust in Christ.
I hope you live up to this extremely high standard yourself and daily lay down your life for your neighbour in imitation of Him.
Thomas (the Apostle) was not rejected because of his failure to beleive but was rather lovingly and patiently shown the evidence that would satisfy him.
As for me, my brother, I trust in the grace of Christ, don't throw stones in my glass house and humbly bear with those that I disagree in the cause of unity in the name of Christ Jesus.
Scripture says such people are to be rejected. "A man that is an heretic after the first and second admonition reject (Titus 3:10)."
The word heretic means a divisive one. A divisive person is the proud one who tries to enforce His opinion on others without regard for the unity of the church. He must be removed before the whole is divided.
Pretending that people who reject Christ are part of the body of Christ is not unity.
What is the rejection of Christ?
I seem to remember that a denomination from Rome practiced the paying of indulgences for salvation at one point. Is this not a rejection of the efficacy of the work of Christ on the cross for salvation?
Christ himself said this: Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up, that everyone who believes may have eternal life in him.”
Those who reject what Jesus revealed to the apostles and passed on to their successors have severed themselves from the body of Christ. I wish Luther and Calvin had heeded your warning not to attempt to split the body of Christ. What they did was very wrong and a very serious sin.
Denominilisation was well entrenched by the time of Luther and Clavin and most notably happened at the point where the Bishop of Rome chose to follow the spirit of a proud and foolish man rather than follow the humble, loving and gentle Spirit of Christ Jesus our Lord.