It is worth mentioning that, under this definition, having doubts is not heresy. And there is a spectrum along which we can exist without falling into heresy (though we might be on the road toward heresy while on this spectrum). It is certainly possible to hold opinions whose logical consequences are incoherent with a truth of the Christian faith (or several truths thereof) without understanding that incoherence or having the ability to recognize it. Heresy is an intentional thing, and we cannot be heretics without the intention to believe something contrary to the faith, though we can certainly be heretics without understanding that we are heretics.
One thing that can be learned from a historical study of heresy is that most people who choose a heretical position do so with the best of intentions and often firmly believe that their heretical belief is perfectly coherent with the truth of the Christian faith. They are generally not malicious and can even be quite virtuous in many respects. While heretics share a denial of a truth of the Christian faith and a belief in their adherence to the truth of the faith, they can differ significantly as to how they arrive at a doctrine contrary to the faith.
There are two common paths taken to the citadel of heresy, and these paths should be familiar to the Christian reader from Sacred Scripture.
"Deuteronomy 4:2 Do not add to what I command you and do not take away from it, but keep the commands of the LORD your God that I give you."
We begin down the first path toward heresy when we take the position that Christianity in its fullness is not sufficient, that we need something more than mere Christianity. This is the path taken by the various Gnostic sects who believed that there was a higher truth beyond the existing early Christian worldview, a truth available to those with special knowledge of the esoteric. This is also the path of the Hermeticists, the 16th century astrologer and philosopher Giordano Bruno of recently renewed fame being one example of this search of the esoteric Egyptian beliefs for a higher truth. This path is the one walked by those who are popularly called Mormons, followers of Joseph Smith who brought what he believed was a new revelation of the truth of Christianity to the United States in the 19th century and tacked on a bunch of new texts to the existing Bible.
Even today, there are certainly examples of people who take this well-worn path toward heresy. Some folks have reached the conclusion that we need some of the insights of Buddhism in Christianity to round it out, that we can create a worthwhile syncretistic religion by combining what we see as the best of both traditions, an approach often taken by the Gnostics who existed alongside the Christian communities of antiquity. Perhaps one might believe that Christianity would be better if it were to involve the increasingly popular practice of yoga or the little known Jain monastic disciplines. Perhaps one might seek to merge Christian beliefs with various Native American cosmologies or theological positions. This is an additive Christianity, a process of tacking on systems of thought incoherent with Christianity and calling it Christianity, but a better Christianity with a higher knowledge or a deeper understanding.
We begin down the second path toward heresy when we take the position that Christianity in its fullness is not necessary, that we can remove some of the parts we find uncomfortable for philosophical or emotional reasons and have a purer Christianity. This is the path taken by Arius and others, to deny that Christ is consubstantial with God the Father because he could not reason his way to the traditional Christian view of the Sonship of Christ. This is also the path of Martin Luther and the Protestant Reformers who followed after him, whether it was taken by removing books from Sacred Scripture to suit his theology as Martin Luther did or by adopting the three Solas (Sola Gratia, Sola Fide, Sola Scriptura) so as to exclude the role of the Christian tradition that had formed the scriptures and provided the concepts of grace and faith used by the Reformers. This path is the one walked by those who were popularly called Shakers, a branch from the Quakers founded by George Fox in the 17th century, Christians who quite seriously took away sex from Christianity, practicing abstinence in a heroic fashion that lead to the collapse of many of their communities.
Even today, we see this path being taken quite often as more and more people describe themselves as Christians without a church or spiritual without a religion, taking more and more away from Christianity until it is unrecognizable as having anything to do with Christ because it is fully compatible with contemporary values, values which are often mutually exclusive with the values of Jesus in the Gospel. This subjecting of the faith to the limitations of the reasoning capacity of the individual often leads to a philosophical religion and can even take us further down the path to universalism, an increasingly prominent view among my contemporaries which suggests that all will be saved, making all religions equally valid in their view. This is a reductive Christianity, a process of taking away from the deposit of the Christian faith until it suits our preferences and priorities and still calling it Christianity, but a better Christianity without all those troublesome parts that challenge our sensibilities.
In many cases today, we are tempted to walk both paths toward the citadel of heresy, adding to Christianity what we find valuable in other religious traditions and in secular contemporary values while at the same time taking away or downplaying those parts of Christianity that are counter-cultural and seen as unenlightened and irrational. The result is a customized Christianity, a Christianity which has been reshaped in the image of the individual who has fashioned it to meet their individual preferences.
It is an additive Christianity in that they have added their own preferences to it and fashioned it in their own image, and it is a reductive Christianity in that they have taken away the parts of Christ's message that challenge them, leaving only the platitudes that comfort them without asking them to grow. It is a Christianity which at a certain point is no longer Christianity and can only be properly described as a religion of the individual's own making, a Selfianity in which Christ has been pushed out to make room for the Self as the final arbiter of truth.
This is where heresy ultimately leads; in denying a truth of the faith, that faith is made incoherent and thus there is less reason to accept it along with more reason to reject it. In heresy we unravel a thread in the seamless garment of the Christian worldview, gradually tearing away the garment to reveal a Christ without the garment he made for himself and finding a Christ whose teachings are unacceptable to our egos, a Christ from whom we then walk away alongside the rich man of the Gospel so that we can embrace a new religion constructed of our own preferences and pathologies, perhaps remembering fondly a Christ we could not quite accept and perhaps being disappointed by Christ being so unenlightened and irrational.