I am a Christian - meaning I believe and have faith that Jesus Christ is the saviour of my soul. I saw a video before that shows different sects of Christianity and different views as well. It starts with a girl saying Jesus Christ is his saviour in the name of the father of the son and the holy spirit. Then here comes the Pentecostal saying Jesus Christ is the name of God and trinity is not true and you won't be saved when you don't perform the speaking of tongues. Then another guy from Baptist also has a different view, then a mormon has another view..and on and on.. Then there is this stereotypical islam that said it is islam or else, then there's buddhist who practices meditation. The Atheist just said I would rather be an Atheist as these religious groups have conflicts. Then it came into my mind, why are there so many sects of Christian religion? How could I explain this faith to an atheist when another christian has a different view?
Why are there different sects of Christianity?
Well, how long is a piece of string...
This question will probably be closed as either too broad or primarily-opinion-based.
Overall, we could speculate whether it was because of man's sin, God's design or a combination of the two, but perhaps that sort of overview wouldn't really be a very helpful description to your Atheist friend.
Perhaps breaking some of the major splits down into categories of proximate causes would be somewhat helpful:
1. Doctrinal disputes
This is not only probably the most common major reason, but also tends to be the most consequential (both in terms of duration, area-of-effect and difficulty of being overcome). The classic example is the Protestant/Catholic split, but the rise of most new movements (e.g. Baptists, Pentecostals, Jehovah's Witnesses) comes this way. A very old example of this kind of split is the one that lead to the Oriental Orthodox Church going their own way about one and half millenia ago. Disputes can be over very serious things such as soteriology (what makes a Christian/how can someone be 'saved') or comparatively 'minor' issues such as what way Jesus' human and divine natures were combined, or what age it is appropriate to be baptised
2. External events, Ethnic or Geographical separation
When people cite that there are over 33,000 Christian denominations to highlight the utter scandal of Christian division, they usually skate over the fact, that there is a massive amount of replication of denominations per different countries or ethnic groups and sometimes because of events within a country (such as civil wars). The existence of different national (ethnic) expressions within the broader Eastern Orthodox Tradition and, I would argue, the reason for the Southern Baptist Convention are examples of this. This reason, while probably the most common, is almost certainly the least significant: with the advent of modern methods of communication and travel, many of these groups are functionally united or uniting - except of course, where reasons 1. and 3. come in to play.
3. Peronalities and Power plays
This reason may not be as common, but it can still be incredibly potent. Although there were certainly aspects of doctrinal issues between the Western and Eastern churches, one would have to surmise that after being in (at times uneasy) communion for the best part of a thousand years, the actual trigger-point of the Great Schism was due more to personalities and power plays than anything else. Of course after the split, the Catholics and the Orthodox continued to diverge and hardened their positions with dogmatic post hoc rationalizations so that any reconciliation would definitely have to overcome what is now a significant doctrinal gulf. Another example I would put in this category, would be the rise of Methodism - Wesley was content to stay within the Anglican communion, abiding within it's doctrinal framework; it was only decisions made by the Anglican hierarchy not to ordain ministers needed for Methodist congregations in America that triggered the resulting split.