"For as in one body we have many members, and not all the members have the same function, so we, who are many, are one body in Christ, and individually we are members one of another." Romans 12.4-5

John 17:18-21 New Revised Standard Version (NRSV)

"As you have sent me into the world, so I have sent them into the world.  And for their sakes I sanctify myself, so that they also may be sanctified in truth. I ask not only on behalf of these, but also on behalf of those who will believe in my through their word, that they may all be one. As you, Father, are in my and I am in you, may they also be in us, so that the world may believe that you have sent me."  - John 17.18-21

Religion can be a set of beliefs about God or it can be an adherence to the rules of a spiritual teacher. It can be an activity that leads one to behave in an intensely devoted way, or a systematic approach to moral and ethical direction.
The root of the word is Latin. "ligere" meaning tied or bound, and "re" a prefix meaning again. Religion throughout recorded history has been a means of taking people who may have become separated and tying them together again. Those bound together by religion have rules, doctrines, beliefs and practices that adherents believe identify them as distinct from others.
"Religion" is an overarching designation for those practices that keep us distinct. Whether your religious practice is Hindu, Buddhism, Christian, Judaism, Islam, or one of so many others; it is common agreement to certain tenets that keep you connected - bound - to one another in that religious practice, that particular "faith".
Humans seem to be somewhat obsessed with being more correct, more unique, more important than others. It doesn't take long for a faith to become fractured and divided as adherents disagree on what remains "core" belief and how that faith practice is lived out in community and in the world.
Christianity is a good example of this phenomenon, yet by no means is it unique in the experience.
The early Church consisted of small congregations who were encouraged and lead through the teachings of the Apostles, and the letters of faithful followers that circulated among the believers.
As happens, over time differences arose. Eventually conflict came, some congregations identified and connected with the Eastern part of the Church and the Constantinople leadership. Other congregations identified and connected with the Western part of the Church and the leadership in Rome. Since 1054 these two "halves" of the Church have been disconnected from one another.
In the early 16th century there began what came to be called the "Protestant Reformation". Ostensibly it began with Martin Luther who questioned several of the Roman Church's teachings. The main conflict was not unlike the main East/West conflict. The Roman Church insisted that a core teaching for Christian faith must be the infallibility of the Pope. They held that the Pope could never be wrong in teachings, because that office began with Jesus' "appointment" of Peter as the first Pope and the continued work of the Holy Spirit in choosing his successors.
What began as "house churches" two thousand years ago, today exists as myriad Christian faith practices, each holding some unique teaching or interpretation of Scripture that "separates" one from the other. Not all Christian faith practices agree on what is necessary for salvation or for Christian faith itself. As we have become more divided as people, more intransigent and intractable in our belief demands, the possibility that "all may be one" seems more and more remote.
Here is are some questions that lay on my heart. What if the Kingdom of God cannot be fully realized until all of God's disciples are willing to lay aside their demands and come together? Will the day ever come when we accept that we are stronger together than we are in conflict with one another? When we give assent at last in agreement on the core tenets of a faithful Christian life and accept that much of what we demand is "adiaphora" - unnecessary for faith or salvation? Jesus told his first disciples that "wherever two or three are gathered in my name, I am there among them." (Matt. 18.20) Imagine how the world could change of we ALL came together in Jesus' name? What a day that would be!