Why do Christians believe in the book of Mark? Is the book of Mark a make believe book?

Mark is likely the oldest of the gospels. It was written for a Roman audience, and omits many things that might

make sense only to a Jewish audience. It's short and pithy. It is what it is. No need to denigrate it because you think it should have been larger. The other gospels fill in more material, so read them, too.

You think the quotes of Jesus are magical? That they appeared on the pages of the manuscripts independently of what the writer was writing about? Remarkable! Do recall that the authors of the gospels recorded the words of Jesus. If you can't believe the other material, why would you believe the quotes written by the same men?

You seem not to know that Jesus met with the disciples at least twice, once without Thomas, and one with him present. Mark 16:11 clearly records when Jesus meets with them all, including Thomas, as John 20:26-29 records; Mark simply omits the earlier meeting in John 20:19-23. He wasn't writing about Thomas, but about the commissioning of them all to preach the gospel.

You think it's an error when one quote is longer than another? Again, remarkable. Cannot both be true. I suspect Jesus probably said more than was recorded. Does that make *both quotes wrong?

Let the gospel writers tell their story. I'm sorry that you want more, but read the rest of the gospels to fill in. Be prepared to discover that they also did not write everything, but only what they felt necessary:

"And many other signs truly did Jesus in the presence of his disciples, which are not written in this book:

But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name." John 20:30-31.

There is life there; look for it, and don't make up some grievance.

Here are some Bible study tools to consider in your quest.

The Book of Mark is different than the others. http://www.heavendwellers.com/37%20Mark%201381-142...

From The Companion Bible KJV - Appendix 96. The Diversity of the Four Gospels https://levendwater.org/companion/append96.html

From The Companion Bible KJV - Appendix 97. The Unity of the Four Gospels https://levendwater.org/companion/append97.html

Mark 16:14 KJV Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen.

For a detailed chart from The Companion Bible KJV - Appendix 166. The Sequence of Events Following The Lord's Resurrection https://levendwater.org/companion/append166.html

From The Companion Bible KJV - Appendix 168. The Last Twelve Verses of Mark's Gospel https://levendwater.org/companion/append168.html

The book of Mark was not written to Jews, but to Romans and presents Jesus with great power.

No one is mad with you you for being an atheists. You will always find faults with religious books. You have free will to do so....so what's the big deal?

You do know that the other gospels are just copies of Mark that were written later and with extra embellishments added, right?

apparently you don't believe in one Book from Scripture, which probably means that you don't believe ANY of the Word..............that's your choice ; Christians who have been taught Scripture correctly will know otherwise

The four Gospels together record what others may have missed.. !!