Before Jesus’ birth at ~7-2 BCE, there were thousands of different religions around the world from the those of the Native Americans to thoses of the Africans and of course, those of the Far East.
1. Why do they exist?
2. Why did God choose the Jewish people?
3. What could make Christianity true despite thousands of other religions existing?
Mankind were made in the image of God (Genesis 1:27). We inherently know that there is a supernatural Creator (Romans 1:18-20). We were created with a conscience and a sense of moral right and wrong. Humans everywhere at in all times have sought to come to terms with the ultimate reality of the universe as they saw it, and have tried to find ways to behave morally. For these reasons, humans have always created religions as a means to express what they inherently knew about morality and the existence of a greater power.
This explains why virtually all people are religious. Yet we humans are also very sinful and rebellious against our Creator. According to the Bible, we therefore lost our priviledged place with God because of our rebellion. I like to describe the outline of the Bible in this way, and in the process it will answer your second question.
1. Genesis 1 God created everything and it was all good–very good.
2. Genesis 2 God created us in his image in order that he could love us and we could love him and one another.
3. Genesis 3-4 We willfully rebelled against our Creator, creating separation from HIm and from one another.
4. Genesis 5-Revelation 20 God is fixing the probelm.
5. Revelation 21-22. God has fixed the problem and those who have been saved through Christ are back in the Garden.
This was God’s plan, according to the Bible.
He chose a man of faith through whom to work his plan. That man was Abraham, the “father of us all” (Romans 4:16). All who are saved are, like Abraham, made righteous by believing.
God gave Abraham a promised son through whom he planned to bring blessing to the world. Through Isaac and then Jacob and his sons, he created a people to whom and through whom to send the Messiah to save mankind. Deuteronomy 8 and Deuteronomy 7:7 makes it clear that God chose the Jews, not because they were better than all the other people, but because he wanted to save mankind and because of the faith of Abraham. This is why he chose the Jewish people.
As for the truth of Christianity, that is something you will have to decide, of course. What convinces me is what I consider to be the absolutely undeniable fact of the inspiration of the Bible. I believe the Bible is inspired by God because of the fulfilled prophecies, the historical and scientific accuracy of the Bible, the evidence for God working miracles, and the evidence from the personal life of Jesus and the clear evidence for his resurrection. I will attach a couple of articles which are a very slight introduction to the reasons I believe that Christianity is true. Claims of Jesus PPT EvidenceforJesus notes I have much more information than this available. I want to respectfully request that you purchase a copy of my book Reasons for Belief at www.ipibooks.com. I go into quite a bit of detail there on why I believe that Christianity is true. If you have any other specific questions, do not hesitate to ask.