Is it true that Christianity is the most divided religion in the world?
Since the beginning, the Christian Faith has had to deal with heresy and division. Seeing that there are some 45,000 different Christian denominations. The core beliefs of various Christians differ vastly, even to the point of some not being recognized as being Christian whether sacramentally (valid baptisms) or by some doctrinal basis. Christians also have diverse teachings on what constitutes sin and what actions offend God, the nature of God...! One could go on and on. The core beliefs are truly that divergent.
The most detailed level of our taxonomy of global Christianity is Christian denominations, defined as an organized Christian church, tradition, religious group, community of people, aggregate of worship centre, usually within a specific country, whose component congregations and members are called by the same name in different areas, regarding themselves as an autonomous Christian church distinct from other churches and traditions. Denominations are defined and measured at the country level, creating a large number of separate denominations within Christian families and Christian traditions. For example, the presence of the Catholic Church in the world’s 234 countries results in 234 Catholic “denominations”, though these can be further subdivided by rite (e.g., Byzantine or Latin). The typical way for Christians to count themselves is at the local congregational level and then aggregate these totals at the city, province, state, regional and finally, national levels. Individual congregations are not counted as “denominations.” We do make note of the fact that many independent congregations are not a part of any denomination. If those churches were to form an independent network with a name, we would consider them a denomination. Using this method, we report 45,000 Christian denominations in the world in 2019. - How do you define a “Denomination”?
Some of the theological reasons for the division between the Christian denominations are very serious, while other theological aspects are of lesser importance. In any case Christians are not united in a common faith.
Ut unum sint! Our Lord himself prayed that his Church should remain united, but history has shown that Christians have fallen short of Jesus’ desire. “That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.“ (John 17:21)
Sadly and unfortunately many Christians have killed one another over their beliefs over the centuries.
The European wars of religion were a series of wars waged in Europe during the 16th, 17th and early 18th centuries. Fought after the Protestant Reformation began in 1517, the wars disrupted the religious and political order in the Catholic countries of Europe, or Christendom. Other motives during the wars involved revolt, territorial ambitions and great power conflicts. By the end of the Thirty Years' War (1618–1648), Catholic France had allied with the Protestant forces against the Catholic Habsburg monarchy. The wars were largely ended by the Peace of Westphalia (1648), which established a new political order that is now known as Westphalian sovereignty.
The European wars of religion are also known as the Wars of the Reformation. In 1517, Martin Luther's Ninety-five Theses took only two months to spread throughout Europe with the help of the printing press, overwhelming the abilities of Holy Roman Emperor Charles V and the papacy to contain it. In 1521, Luther was excommunicated, sealing the schism within Western Christendom between the Roman Catholic Church and the Lutherans and opening the door for other Protestants to resist the power of the papacy.
Although most of the wars ended with the Peace of Westphalia in 1648, religious conflicts continued to be fought in Europe until at least the 1710s. These included the Savoyard–Waldensian wars (1655–1690), the Nine Years' War (1688–1697, including the Glorious Revolution and the Williamite War in Ireland), and the War of the Spanish Succession (1701–1714). Whether these should be included in the European wars of religion depends on how one defines a "war of religion", and whether these wars can be considered "European" (i.e. international rather than domestic).
The religious nature of the wars has also been debated, and contrasted with other factors at play, such as national, dynastic (e.g. they could often simultaneously be characterised as wars of succession), and financial interests. Scholars have pointed out that some European wars of this period were not caused by disputes occasioned by the Reformation, such as the Italian Wars (1494–1559, only involving Catholics), as well as the Northern Seven Years' War (1563–1570, only involving Lutherans). Others emphasise the fact that cross-religious alliances existed, such as the Lutheran duke Maurice of Saxony assisting the Catholic emperor Charles V in the first Schmalkaldic War in 1547 in order to become the Saxon elector instead of John Frederick, his Lutheran cousin, while the Catholic king Henry II of France supported the Lutheran cause in the Second Schmalkaldic War in 1552 to secure French bases in modern-day Lorraine. The Encyclopædia Britannica maintains that "[the] wars of religion of this period [were] fought mainly for confessional security and political gain."
In the late 20th century, a number of revisionist historians such as William M. Lamont regarded the English Civil War (1642–1651) as a religious war, with John Morrill (1993) stating: "The English Civil War was not the first European revolution: it was the last of the Wars of Religion." This view has been criticised by various pre-, post- and anti-revisionist historians. Glen Burgess (1998) examined political propaganda written by the Parliamentarian politicians and clerics at the time, noting that many were or may have been motivated by their Puritan religious beliefs to support the war against the perceived Catholic king Charles I of England, but tried to express and legitimise their opposition and rebellion in terms of a legal revolt against a monarch who had violated crucial constitutional principles and thus had to be overthrown. They even warned their Parliamentarian allies to not make overt use of religious arguments in making their case for war against the king. In some cases, it may be argued that they hid their pro-Anglican and anti-Catholic motives behind legal parliance, for example by emphasising that the Church of England was the legally established religion: "Seen in this light, the defenses of Parliament's war, with their apparent legal-constitutional thrust, are not at all ways of saying that the struggle was not religious. On the contrary, they are ways of saying that it was." Burgess concluded: "[T]he Civil War left behind it just the sort of evidence that we could reasonably expect a war of religion to leave."
For unity amongst Christians to be genuine, we must not only learn to understand one another, we must also learn to ask pardon and forgiveness of those of other denominations we have wronged. We must search for the truth, while admitting our own shortcomings and faults.
No other world religion can boast as having so many denominations or splinter groups as Christians have.
Islamic schools and branches do come close to the Christian figure of 45,000 denominations.

Diagram showing the various branches of Islam: Sunnīsm, Shīʿīsm, Ibadism, Quranism, Non-denominational Muslims, Mahdavia, Ahmadiyya, Nation of Islam, and Sufism. - Islamic schools and branches
Judaism is likewise not as divided as we are. Besides that the Jewish population is nowhere close to that of Christians worldwide.
Jewish religious movements, sometimes called "denominations", include different groups within Judaism which have developed among Jews from ancient times. Today, the most prominent divisions are between traditionalist Orthodox movements (including Haredi and Religious Zionist (Dati) sects); modernist movements such as Conservative, Masorti and Reform Judaism; and secular or Hiloni Jews.
Buddhism has it’s diverse groups of doctrinal diversity also.
The schools of Buddhism are the various institutional and doctrinal divisions of Buddhism that have existed from ancient times up to the present. The classification and nature of various doctrinal, philosophical or cultural facets of the schools of Buddhism is vague and has been interpreted in many different ways, often due to the sheer number (perhaps thousands) of different sects, subsects, movements, etc. that have made up or currently make up the whole of Buddhist traditions. The sectarian and conceptual divisions of Buddhist thought are part of the modern framework of Buddhist studies, as well as comparative religion in Asia.
From a largely English-language standpoint, and to some extent in most of Western academia, Buddhism is separated into two groups: Theravāda, literally "the Teaching of the Elders" or "the Ancient Teaching," and Mahāyāna, literally the "Great Vehicle." The most common classification among scholars is threefold: Theravāda, Mahāyāna and Vajrayāna.