Does Islam and Christianith have the same start?

Yes they are related: One of the messengers of God Prophet Ibrahim had two sons Prophet Isaac and Prophet Ismail. They were both also messengers of God, from the descendants of of Prophet Isaac many prophets came including Prophet Isa (Jesus). So Jesus came with Christianity which was the Islam of that time. Afterward nearly 600 years later Prophet Muhammad came as the descendant of Prophet Ismail and he brought an end to prophet hood and brought the Islam of that time and all time thence forward. So actually even the Jews are related because they were followers of Prophet Moses one of the descendants of Prophet Isaac. So the God is same the difference is that the earlier people of Islam failed to recognize the last Prophet of which was fore told to them so they became part of another religion as Jews and Christians.

The Israelites answered G'd's call first. Then christianity codefied their christian testament around 300 CE (the common era) The Islamic people did not start their religion until the mid 600s CE. So the Hebrew faith predates Islam by thousands of years. You cannot say that they are the same. They took Hebrew ideas as did the christians and incorporated them into their book and faith that is all. I don't think that christians would like you to say that they update their book since they say it is the word of G'd. Muslims also say that their book is the word of G'd. Shalom~Adoniah