Christians believe in monotheism, because Jesus taught this.  He taught us to love and believe in the one true living God.

The word Trinity does not appear in the Bible, but references to the Father, Son, and Holy Spirit as one can be found in many places.

Consistent with the idea of being one with the Father, Jesus had the power to forgive sins, and had no problem accepting worship from others.

Jesus compared himself to "living water".   It's interesting that water can exist in 3 forms... water, steam, and ice... yet all 3 forms are all made up of the same elements.

So it is with the Father, Son, and the Holy Spirit.

Jesus also taught in parables about the divinity of the Messiah, or as he preferred to be known, "The Son of Man".   This title was used to demonstrate humility and servanthood to us, but not to relinquish his divinity or position as "Emmanuelle" (God with us).

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Mark 12:28-37

The Most Important Commandment
28 One of the teachers of the Law of Moses came up while Jesus and the Sadducees were arguing.   When he heard Jesus give a good answer, he asked him, "What is the most important commandment?"   29 Jesus answered, "The most important one says: People of Israel, you have only one Lord and God.   30 You must love him with all your heart, soul, mind, and strength.'   31 The second most important commandment says: Love others as much as you love yourself.' No other commandment is more important than these."   32 The man replied, "Teacher, you are certainly right to say there is only one God.   33 It is also true that we must love God with all our heart, mind, and strength, and that we must love others as much as we love ourselves.   These commandments are more important than all the sacrifices and offerings that we could possibly make."   34 When Jesus saw that the man had given a sensible answer, he told him, "You are not far from God's kingdom."  After this, no one dared ask Jesus any more questions.

About David's Son
35 As Jesus was teaching in the temple, he said, "How can the teachers of the Law of Moses say that the Messiah will come from the family of King David?   36 The Holy Spirit had David say, The Lord said to my Lord: Sit at my right side until I make your enemies into a footstool for you.'   37 If David called the Messiah his Lord, how can the Messiah be his son?"  The large crowd enjoyed listening to Jesus teach.

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It's not a coincidence that Jesus follows his teaching of having and worshipping only one God with this question: "If David called the Messiah his Lord, how can the Messiah be his son?"

Jesus submitted to The Father as an example for us all... but he also taught us about the concept of God's ability to be omniscient and omnipresent, and He had no problem understanding what is difficult for us to see.   He knew Zacheus' name in the tree, and the inner details of the life of The Woman at the Well, and the thoughts of his disciples and others even when they were not speaking.

Now consider this... Jesus is speaking about the date and time of his return and the end of this age:

Mark 13:32
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32 No one knows the day or the time.   The angels in heaven don't know, and the Son himself doesn't know.   Only the Father knows.
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I must admit, that I have struggled to explain why Jesus didn't know the answer to this question.

The Life Application Study Bible has this to say:

"When Jesus said that even he did not know the time of the end, he was affirming his humanity.   Of course God the Father knows the time, and Jesus and the Father are one.  But when Jesus became a man, he voluntarily gave up the unlimited use of his divine attributes."

The emphasis of this verse is not on Jesus' lack of knowledge, but rather on the fact that no one may know the answer to this question.   It is God the Father's secret to be revealed when he wills.   No one can predict by Scripture or science the exact day of Jesus' return.   If Jesus in his human form were granted the ability to recall this information and then to speak of it, it would most certainly have been recorded in scripture.   But it is not the Father's will that this be known.