Why do Christians think their Jewish God is real?

Because all of the prophecies in his word - the Bible has come through and continues to come through.

my Mom told me so.

why would she lie?

Jewish God? Are you still in school?

I have met Yeshua ("Jesus") a number of times over the past 25 years. He is real, He is alive, He has spoken to me, and I have spoken to Him. Each encounter was unexpected and unique. Search YouTube for "Jesus encounter" and you will see He has appeared to atheists, Jews, Muslims, etc...

John 8:23-24 Yeshua said to them, “You are from below; I am from above. You are of this world; I am not of this world. I told you that you would die in your sins, for unless you believe that I am he [the messiah, the Son of God] you will die in your sins.”

Matthew 10:37-38 [Yeshua speaking] Whoever loves father or mother more than me is not worthy of me, and whoever loves son or daughter more than me is not worthy of me. And whoever does not take his cross and follow me is not worthy of me.

Is Yeshua the most important thing in your life? How much time do you spend with Him every day? Is He your friend?

To "take up your cross" is to deny the desires (not the needs) of the flesh and be obedient to the laws and commandments of God. As you develop a relationship with Yeshua ("Jesus") you will hear from the Holy Spirit (Dreams, Visions, Words of Knowledge, Words of Wisdom, etc...) You must be obedient to what the Holy Spirit communicates to you without hesitation. If you are asked to do something, or told not to do something, the immediate answer is "Yes Lord", worry about the details later - He will lead and guide you in accomplishing anything you are asked to do as you do it.

Spend time in prayer every day (praying out loud). Here are prayers I say every day. Modify them as you see fit for your situation and circumstances:

https://www.dropbox.com/s/li9k4kpvhvfx9yc/Daily%20...

Spend 30 minutes or more every day reading your Bible (reading out loud). Start with Matthew and read to the end, then start at the beginning. I use the ESV, but the King James (not the "New" King James) is the most accurate translation as it is the ONLY translation that uses the Textus Receptus manuscripts.

If you are willing to set aside the things that take up most of your time (TV, movies, gaming, Facebook, Internet, etc... you know what you spend most of your time doing), and dedicate three hours every evening to the Lord, within a few months if not sooner you will see a mighty move of God in your life. Develop the following routine and stick to it consistently:

Spend the first hour praying in tongues. Praying in tongues (out loud) is probably the most important thing I have learned to do. Since I started doing this I have had numerous dreams, visions, and even encounters with Yeshua. Here is a document that will help you understand and get started:

https://www.dropbox.com/s/5pibf1bwoqmoo74/Pray%20i...

Spend the second hour waiting on the Lord quietly. This will help you learn how:

https://www.dropbox.com/s/57n603ps4tv62ri/Wait%20A...

Spend the third hour singing (out loud) praise and worship songs. These songs should mention the blood of Jesus, the cross, and/or attributes of the character and nature of God. Here are a few of my favorites (you can set it to shuffle play so they play in random order):

https://www.youtube.com/playlist?list=PL2kjGV_3m9r...

In addition to that I like to listen to "soaking" music at a low volume while praying, reading, or waiting on the Lord. Here is the music I use:

https://www.youtube.com/playlist?list=PL2kjGV_3m9r...

God does not have a nationality.

Because they have been properly and conveniently brainwashed by priests, pastors, preachers and their fairy tale book. 

Yes for me I know he is real because I see him a lot of times! At school, and a lot of other places to!

They do not regard the living God as being "Jewish" and they know that He is real through Jesus Christ, because of who He is and what He has done. John 14:6 "Jesus said to him, “I am the way, and the truth, and the life; no one comes to the Father but through Me." An exclusive claim that countless people have found and experienced to be true.

How could people like Adam, Enoch, and Noah who lived hundreds of years before the Israelites ever existed have followed a Jewish God?

You have a big problem calling God a "Jewish God" to christians because the Jewish race didn't exist in pre-Abraham days and for the last 2000 years the Jews of Israel have not been God's chosen people.

Christians/ Gentiles (world) including individual Jews are God's people today.

Childhood indoctrination is the usual case