The Holy Quran was revealed to Prophet Muhammad (peace and the blessings of Allah be upon him) over a period of 22 years. The verses were written down and memorized by hundreds of people as soon as they were revealed.
God promised to protect the Holy Quran Himself:
Verily, We Ourself have sent down this Exhortation, and most surely We will be its Guardian. (15:10)
Not an iota of the Holy Quran has changed in 1400 years.
Islam is to be the religion of mankind till the end of time, and the Holy Quran is to be the spiritual guidance for mankind till the end of time.
The Holy Quran is the word of God.