OK, I have a question about God's messenger...

Hello, I don't know if you're thinking about becoming a muslim or you have just a regular research but I'll try to explain you why we believe he's a prophet.
Muhammad, peace be upon him, is truly one of the Allah's prophets. He received Quran, the only Book which says the same as it was when he received it. He's the last prophet of God.
His father died before his birth and his mother died in his early ages. He was raised by his grandfather and after that his uncle.
He became merchant, one of rarely merchant who was trustworthy, so people called him al-Amin.
He was illiterate when he received Quran. First words he received were "Read in the name of your Lord".
His miracle was Quran. In that period, there were a lot of poets and writers but no one couldn't write one sentences similar to Quran. His miracle was something that dominated in that time, just like Musa, peace be upon him, received "magical" stick in a period of many "wizards" who were believed him.
When he needed help, God always helped him, like on Battle of Badr and on Hijra. On Battle of Badr there were about 300 muslims and mushrikeen but muslims won anyway with Allah's help. On Hijra, when he and Ebu Bekr hide in cave, but the grace of Allah, at the entrance of the cave spider weave its network and pigeon built a its nest few moment before enemies came to entrance of the cave.
There's so much more for tell about our prophet. If you have more questions or issues with my answer, fell free to ask me. :)

"Before you, we also sent Messengers who were men We inspired.... We did not give them bodies which did not need food, nor were they immortals." (The Qur'an, The Prophets, v. 7-8)

"That has been Our method in the case of Our Messengers whom We sent before, and you will not find Our method to change." (The Qur'an, Bani Israel, v. 77)

So from the Qur'an we can see the way to recognize Messengers/Prophets of God - those with divinely inspired messages of great Truth. All Prophets have also fulfilled Prophecies of their coming from the previous religions - Christ fulfilled many Jewish prophecies, Muhammad fulfilled Christian and Jewish prophecies found in the Bible, etc. Each religion claims their messenger/prophet was the last, but each religion also has expectations and prophecies of a prophet to come in the future (Messiah, return of Christ, al-Mahdi, etc.). So it seems there will be further Prophets, although most do not seem to want it to be so :).

Indeed, I would encourage you to expand yourself even further beyond Islam to study other religions that also have great truth and teachings. For example, the Baha'i Faith, whose Prophets (Bab/Baha'u'llah) came in the 1800s, has a great teaching, and Bab and Baha'u'llah fulfilled many Islamic Prophecies of the return of Isa (Christ) and the Mahdi.

There is another, very new teaching, brought by a man named Maitreya, who I believe is also a Prophet. He has fulfilled prophecies from all world religions and cultures, more than any before Him. His teaching is of unification of all religions, and that all are from God and each a part of the full truth. I believe He fulfills the many expectations of the Prophet/Messiah/Christ/etc. to come in the end times, and that His Teaching is of profound truth and will change the world. I encourage you to study it also, and prove to yourself if it is also from God, and in the same spirit as the beauty you have found in Christianity and Islam. Maitreya's teachings can be found here: www.maitreya.org

I am also embedding a video which gives a brief introduction to the base of Maitreya's teachings.

I hope this helps.

Yes we muslims believe that Muhammad (p.b.u.h) is the messenger of God just like prophets David Musa Jesus peace be upon them all .Actually He is the seal of the prophethood.All the the prophets were provided with miracles so that people may belive them like prophet Musa was given staff which used to become serpent when put on ground and prophet jesus could heal blind person cure leprosy patients bring life to the dead ones only by Allah's help likewise muhammad was given miracles.Once the people of arabia told the prophet that they would only believe in him when he will break the moon in 2 halves by his finger. He then directed his finger to the moon as told by angel gabriel to do so the moon was split into 2 halves later the 2 parts joined together again and today we have got proof that moon surface has crack.and the greatest miracle of Muhammad (saw) is the holy quraan .(plz read miracles of quraan by haluk nurbaki english translation) how could an illiterate man of 14th century know all these things(physics,chem,bio,geology,etc concepts) may Allah guide u to the peace n me too aameen