Why is rejection of the Trinity heresy, and not just wrong? (Protestant perspective)

It is "just wrong". It simply means "you are wrong (according to what we accept as Truth) so you can't call yourself (be in communion with) one of us". And yes, believing in predestination of salvation does make you a heretic in the eyes of the Catholic and Orthodox churches.

Protestant traditions are sometimes a little more flexible and forgiving when it comes to differences in belief, but there are certain things (i.e. Trinity, Salvation by Grace, etc) that are so core to Christianity that most* Christians will consider those with contrary beliefs (i.e. Momons) to be heretics, even if they don't use that word.

*for some definition of "most"

For me this boils down to idolatry. If you worship a none-triune God, you have constructed a god that is not the same God as the God of the Bible. So it's heretical rather than just simple doctrinal disagreement because you are not even worshiping the same God. It doesn't matter if you call it god, it's not God. This goes against the first commandment. But more than that I can say this qualifies as blasphemy against the Holy Spirit to say that the Holy Spirit does not exist or is not God. Mark 3:29 states "but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”

I would like to point out that it is not common to jump to the conclusion that someone is a heretic flatly because of their lack of understanding of the Trinity, this can be the case and often is; however, more clarification may help in this discussion.

That being said, the Trinity is something that a Saved believer will come to affirm as it is Truth; however - upon being Justified, there is not necessarily outpouring of all understanding bestowed upon the believer per se'.

It is possible to become a sincere Christian, and to then grasp onto more Godly concepts as the Lord works out His plan of Sanctification for the believer. If a newly Born Again adherent is unaware of the Truth of the Trinity at the onset - he is possibly not engaging in active heresy; it may be the case that he or she has not had that aspect of God's Nature revealed to them as of yet. I would contend that this is a rare case, but is certainly possible.

That being said, it is proper for Christians to affirm the Trinity, as it is clearly taught in Scripture.

Many others have done well to make valid points concerning this topic, but I'd like to take the most simple and effective approach to answering. We can go all over the Scriptures and see evidence that the Trinity is a sound and rightful Doctrine, in both the Old and New Testament.

Unfortunately, I do not have the time presently to show in an exhaustive manner all of these evidences here; also I want to address the topic at hand rather than provide all evidence that it is in fact a Truth.

Also, although it can be clearly shown to be a Truth from Scripture, it should be understood that it is not possible for man to fully comprehend it in totality. It is, however, clearly a reality, despite our inability to fully ascertain the inner workings of God's Nature of Being (Trinity).

I will conclude with my answer by demonstrating that it is simply not possible for a Born Again believer to maintain in some ongoing fashion a denial of the Trinity by using Christ's own words:

Matthew 28- The Great Commission

16 But the eleven disciples proceeded to Galilee, to the mountain which Jesus had designated. 17 When they saw Him, they worshiped Him; but some were doubtful. 18 And Jesus came up and spoke to them, saying, “All authority has been given to Me in heaven and on earth. 19 [e]Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, 20 teaching them to observe all that I commanded you; and lo, I am with you [f]always, even to the end of the age.”

Christ Himself, who is God, not only affirms this aspect of His own Nature of Being, but ties in His direct mention of it with the Great Commission; a foundational command which all true Christians follow.

1) Christ has ALL Authority in Heaven, and on Earth. (We can know He speaks Truth about all matters as He is God Himself, the second person of the Trinity) 2) It should be understood that God would never command His own followers to baptize in any other name than His own.
3) Clearly the Trinity is taught by Christ Himself, therefore, all true Christians - upon this truth being revealed to them will affirm it.

I hope this helps, but a small bit of further clarification may be required, while I explained it may for a time be unclear for a newly Saved believer (simply wrong/uneducated on the Doctrine), ongoing rejection of the Truth of the Trinity would and should be considered Heresy.

• As a side note I'd like to add that Catholics do in fact affirm the Doctrine of the Trinity.

Narnian's write up is quite well done, but to try and distill it down a bit, the core issue is criticality of belief. Typically in Christian circles this revolves around salvation and the nature of the Trinity is considered by most to be critical to the nature of salvation.

Effectively, a Christian would believe someone to be a heretic if what they say is so far removed theologically as to be an incompatible belief, whereas they may see someone as simply wrong (that is, incorrect, not morally wrong) if it is in an area seen as non-critical. (For a soft-ball example, most Christian's don't see it as a critical deficiency in someone's theology if they have a different view of the book of Revelation.)