Religion, by dictionary definition is
1. a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
2. a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects: the Christian religion; the Buddhist religion.
Christianity certainly meets the definition because it has all of these things. It has a set of beliefs regarding creation. It has devotional and ritual observances. It certainly has a moral code. And while some are more “fundamentalist” than others, all Christians share in belief in God and Jesus, in salvation unto eternal life, in the general outline given in the Apostle’s Creed and (normally) the Nicene Creed, and so on. Christianity certainly has plenty of creeds, doctrines (teachings), catechisms, and Sunday School lessons. So Christianity has all of these things, but it is not any of these things. At least God does not define the faith by any of these things.
Religion, via Wikipedia:
Religion is a collection of belief systems, cultural systems, and world views that relate humanity to spirituality and, sometimes, to moral values.[note 1] Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to explain the origin of life or the Universe. They tend to derive morality, ethics, religious laws or a preferred lifestyle from their ideas about the cosmos and human nature. According to some estimates, there are roughly 4,200 religions in the world.
Many religions may have organized behaviors, clergy, a definition of what constitutes adherence or membership, holy places, and scriptures. The practice of a religion may also include rituals, sermons, commemoration or veneration of a god, gods or goddesses, sacrifices, festivals, feasts, trance, initiations, funerary services, matrimonial services, meditation, prayer, music, art, dance, public service or other aspects of human culture. Religions may also contain mythology.
Again, Christianity certainly qualifies here. But again, having these things: beliefs, cultural and world views, spirituality, moral values, narratives, symbols, traditions, sacred histories … clergy, holy places, scriptures, rituals, sermons … does not mean it is any of these things or even all of these things put together. None of them answer the question, “What is Christianity?” All of them may have a place and some relative value, but none of them really defines Christianity. Indeed, these outward things only make sense when you get at the root reality of “the Christian Faith,” and in that sense I would argue that all of these things, dictionary and encyclopedia alike, miss the point of what Christianity is because Christianity has these things but it is none of these things.
The definition of Christianity is not found in any of these outward things, because it is only found in inward things. In that sense, it is not a religion at all. It is instead a relationship. Beginning with Genesis and working all the way to Revelation we see the testimony, much of it eyewitness testimony, of the relationship between God and ourselves. John Calvin, the reformer said the Scriptures of the Old and New Testaments tell us about God and ourselves. And that testimony, even eyewitness testimony, has not ceased with the Revelation given to John. If anything it has increased in the last 2000 years. Roman Catholics call those who have shown in manifest ways that they were in an extraordinary relationship with God, Saints. The reformers countered by saying that all who are called into a relationship with God are saints, whether they manifest that relationship in some way or not.
The point is, in a very (most) real sense, Christianity is not a religion. It is a relationship between us (personally and corporately), and Almighty God through Jesus by the power of the Holy Spirit. It is nothing less and nothing more. Everything else associated with the faith, every outward thing by dictionary or encyclopedia is window dressing.