Thanks for the excellent question.
Before answering your main question, perhaps I can clear up your last point. So, we have Judaism, Christianity, and Islam.
Jewish people believe that they are the chosen people and await a Messiah to come to bring salvation. As their main text, the Jews believe in TaNaKh (Torah, Nevi'im, Ketuvim, which collectively forms the Christian Old Testament).
Christians believe that God has already sent us that Messiah, Jesus Christ. Salvation is available by believing in Jesus. Christians believe in the TaNaKh (called the Old Testament) and the text written by Jesus's disciples and other followers (called the New Testament).
Muslims (Islam believers) believe that Jesus was NOT the Messiah, but just a prophet, just like Muhammad. However, Muhammad is the last prophet, and his message holds more weight. For that reason, the Muslims believe in the Christian Old and New Testaments but emphasize the Quran, which is inspired by Muhammed and explains away some conflicts with the Christian faith.
So, which is correct, Judaism, Christianity, or Islam? I am one-sided on this question because I am a follower of Jesus, and I can point to many reasons why Christianity is correct, but I'll leave it to you to gain confirmation from God.
To answer your main question, the Old Testament alluded to a coming Messiah who would reverse the curse that started in Genesis. The curse caused separation from God because of sin. The Messiah would resolve that separation issue.
The Old Testament revealed how that sin issue could be resolved. It was by having a living being (someone or a live animal with blood) die instead of the person. The death of a living being would atone for the sins of another person. See Leviticus 4.
The living being must be young (for example, see verse 3, "a young bull") because that animal must be pure. Only a sinless creature can atone for a sinful person. Then, here is the verse where it shows that Jesus took the place of an animal and did it for good.
How much more will the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify our conscience from dead works to serve the living God. Hebrews 9:14 (ESV)
Now, in some parts of the Old and New Testament, Jesus is portrayed as a prophet. Indeed, Jesus operated as a prophet by hearing from God and proclaiming what He heard. For example, Jesus is referred to by the people as a prophet (Matthew 21:11; Luke 7:16; John 4:19), and Jesus refers to himself as one (Mark 6:4). Moses said about Jesus as the one God will send to tell what God intends (Deuteronomy 18:18, then confirmed in the New Testaments, Acts 3:22, Acts 7:37). So yes, Jesus is a prophet.
Yet, the more significant promise that Jesus fulfilled was that of a Messiah. Only a sinless living being can meet that need. Can any human being be without sin? Many verses, including Romans 3:23, say no human can be without sin. That is why God had to come in human form, whom we call Jesus, to die as a living being to complete the sacrifice once and for all. (And for your information, the prophet did not become God; He was God. See John 1).
Back to the start:
Jews still await the coming savior who will restore their relationship with God.
Christians believe in Jesus, the savior who has already completed the atonement for our sins. As a result, Jesus now lives with the believers through the Holy Spirit.
Muslims do not believe in the atonement of their sins by Jesus (Jesus was just another prophet). Muslims can be saved by performing proper deeds known as the five pillars (Profession of faith, Prayer, Alms, Fasting, Pilgrimage). As long as they can keep this perfect law (some say when a Muslim does more good than bad), they will go to "paradise" (Jannah).
I wrote a little bit longer than I usually do. I hope this answers your question and gives you some things to think about. I would be happy to guide you further if you need any clarification. If you write back, it will be routed back to me so we can dialogue further.
I wish you the best in your journey of curiosity. I hope you'll find Jesus to be your Lord and Savior.
Many blessings!
Des Moines, IA
