How can I answer atheists and Muslim’s arguments like this:  1.) If Christianity is the truth, why there are so many sects and schisms? Imagine thinking you have the truth when you’re just a part of sub sect 26-times schism offshoot of another sect. Christianity has already changed from believing Jesus as a prophet into Jesus as God.  2.) There has been no unified Christianity since the beginning of it. Even the case is the same for larger sects. They are all different. So there is no clear teaching and we can’t know what sect is the right sect. Therefore, Christianity is not the truth.  Note: Didn’t Islam had schism too? Between Sunni and Shia? And if I’m not wrong, Shia had different sects.

Whether the Bible is inspired by God or not is neither proved nor disproved by the number of sects within Christianity.  The same is true for Islam.  There are a number of sects in Islam, obviously, but, to be honest, not as many as there are Christian sects.  But surely the Muslim would not dare to claim that Christianity is disproved by the fact that there are multiple sects, given that this would also prove that Islam is not the truth.  I am sure that no sincere and honest Muslim would use this argument (but some disingenuous ones might)

Why there are so many sects in Christianity is more a question of history than of truth.  Up until 1500 AD, there were only about four or five Christian sects, making it roughly as divided as Islam is today. This is worth noting because in AD 1500 Christianity was slightly older than Islam is today, with about as many major schisms as Islam has had up to today.  It was only after the Reformation that so many Christian sects arose.  This was due to the political changes that occurred during the Renaissance and afterward, as political freedom and democracy came to Europe.  It was a change in the political situation that resulted in believers having control over what form of Christianity they favored.  This does literally NOTHING to establish whether the gospels are reliable, whether Jesus is the Messiah and the Son of God, or whether the Bible is true.  This is a red herring argument, as any Muslim or atheist ought to know.

Which is the “right” sect?  The one that accurately reflects the teachings of Jesus. It is the one which teaches the Bible and the Bible only as the source of truth.  It is the one which teaches the means of salvation the same that the apostles taught it.  It is the one which calls people to live the life that Jesus demonstrated.  More than one sect of Christianity meets all of these criteria.   Which is the “right” sect of Islam?  Again, this does nothing to change whether Islam or Christianity are from God or not.

By the way, the implication of your question is that there was a time when Christians believed that Jesus was a prophet but not God.  This is a falsehood.  It is simply not true.  A large majority of all Christian believers from day one have believed that Jesus is God.  This is incontrovertible fact.  Have there been dissenters to this belief, then and now?  Yes.  There have always been heretical doubters, but the facts of history is clear, that this was never the majority, and the critics saying differently does not make it true.

John Oakes