Was there any other form of Christianity besides Catholicism between Christ and 1500?
Yes. In Africa, the Coptic (Egyptian) and Ethiopian Orthodox Churches have existed since early Christian times. The Eastern Orthodox Church which developed national churches in Europe is as old as the Catholic Church. Both of these were part of the same worshipping community before an unfortunate split. There were Christians in India (the Thomas Church) from the beginnings of Christianity. There were even independent Christians in China prior to 800 C.E. (A.D.). The Waldensians who still exist as a Christian community were “Protestants” hundreds of years before the Protestant Reformation. Some very early forms of Christianity were suppressed in the early centuries by people in power who tried to silence them by calling them “heretics.” There were many expressions of Christianity prior to 1500. Some of those groups are well known but others less so.
(Note: While attributed to Holly Hearon, this response actually comes from Dr. Hearon's colleague at Christian Theological Seminary, Lorna Shoemaker)
There were many forms of Christianity before 1500. Indeed, in the first few centuries of global Christianity one could appropriately speak of Christianities. Gnostic Christians, Donatist Christians, Pelagian Christians, Arian Christians, Monophysite or Jacobite Christians, Nestorian Christians, Coptic Christians, Montanist Christians – and more - populated the religious landscape of the first several centuries following the earliest communities that gathered around Jesus and the stories about Jesus.
Some were persistently suppressed by gradually more normative Christian expressions, like those that centered on Rome and Constantinople that claimed the attention and patronage of the later Roman emperors. Some disappeared as Arab armies brought Islam into North Africa and the Iberian Peninsula or, later, into the Byzantine Empire.
Groups differed in theologies and in practices. Some held differing understandings of who Jesus is or of how God the Creator-Jesus-the Holy Spirit are related. Some had differing understandings of how grace operates in human lives. Some exacted differing standards of behavior for their members. All were attempted to be faithful and to understand the same questions, used essentially the same scriptures – although in differing languages – hoped to spread their understanding of the Gospel message.
Prior to the Reformation, there were (and remain) numerous other kinds of Christianity. In Eastern Europe, the major tradition is Eastern Orthodoxy. The schism between Eastern Orthodox and Roman Catholic traditions is often dated to 1054, but the process was more complex and went on for some time.
Elsewhere, there is the Coptic Church, the Ethiopian Church, the Monophysite Christians of Iran/Iraq, the Mar Thoma Christians of south India, and still others.
In the early 4th century the Roman Emperor Constantine legalized Christianity, which up till then had been illegal in the Roman Empire. Constantine also decided that the Roman Empire was too big to govern from a single capital, Rome, so he founded another capital further east and named if after himself: Constantinople (“Constantine City”; present-day Istanbul). Soon enough, the Roman Empire started to split into two: the Western Empire, governed from Rome, with Latin as its native language, and the Eastern Empire, governed from Constantinople, with Greek as its native language.
In A.D. 476 the Western Empire collapsed, leaving only barbarian tribes, and the Pope in Rome as its spiritual head. The Eastern Roman Empire continued to flourish, with the Eastern Roman Emperor as its temporal head and the Patriarch of Constantinople as its spiritual head. In A.D. 1056 the estrangement and incomprehension (they spoke two different languages) between the Western and the Eastern Church grew so great that they excommunicated each other. Thus from A.D. 1056 until 1517 (Martin Luther) there were (at least) two Christian churches: the Roman Catholic Church in the West, with Latin as its main language, and the Eastern Orthodox Church in the East, with Greek as its main language.
Since then, of course, the Protestant Reformation has split the Western church into even more denominations; and the Eastern Church has divided into a number of more or less ethnic churches, each speaking its own language: Greek, Russian, Syrian/Lebanese, Ukrainian, Armenian, American, etc. etc. Only in the 20th century has the Ecumenical Movement tried to bring all these Christian denominations back at least into conversation with each other.