Because Scriptures teach it.
The divine Word (also called the eternal Son of God) took on a fleshly bodily form. The theological term for this is, incarnation.
This is what John 1:14 says: “And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth.”
In other words, divinity came to be with humanity. This happened in the womb of Mary. Not by human will or biological reproduction (as some Muslims think) but by supernatural miraculous means.
When this happened, He did not cease being God. He was God before His incarnation and He is still God after His incarnation. Jesus was fully human and fully God.
How can Jesus be both man and God?
Jesus has two distinct natures. (This is called the Hypostatic Union).
” Believe me that I am in the Father and the Father is in me..” John 14;11
The Bible clearly teaches of both the humanity and divinity of Jesus. Muslims are all too familiar with the verses that describe Jesus as being a man. Sometimes they point out those verses to Christians. But we know Jesus was a man. No Christian has ever denied Jesus was a man.
However every Muslim either deny or try to misrepresent and misinterpret verses that teach Jesus is God.
For example: John 1:1-3 tells us about His eternal existence and that He is the divine Word through which everything was created.
He fulfilled hundreds of prophecies from the Old Testament prophets.
(e.g. Isaiah 7:14, 9:6, Isaiah 53, Jeremiah 31:31, Psalm 22, Psalm 110, Daniel 7:13-14, Micah 5:2, Genesis 3;15, 12:3, etc, etc… literally hundreds.)
He had the authority to forgive sins (Matthew 9:6).
He received and accepted worship. (Matthew 14:33)
The Bible clearly teaches the deity of Jesus Christ. Muslims take what the Bible says about His human nature but will not accept what the same Bible declares about His divine nature.
Muslims object by saying, “Well, God is not a man.”
We agree. God is NOT a man. God came as a man to the earth He created for a brief 33 years. Just because the INVISIBLE came to the earth in a visible form does not mean he ceased to be God.
In Islam, the angel Jibreel came to Mariam in the form of a man. Did Jibreel cease being an angel when he took on the form of a man before Mariam?
The true Light who gives light to every man was coming into the world. He was in the world, and though the world was made through Him, the world did not recognize Him. He came to His own, and His own did not receive Him.
Muslims also argue, “God does not change.”
Once again, we agree. God did not change at the incarnation. His divine nature remained divine, even though He took on the form of a human.
In Islam, Allah descends to the lower heavens to hear the prayers of Muslims. Do Muslims believe this means Allah changed? Of course not.
Jesus descending to the earth does not mean God changed.
Jesus said: “For I have come down from heaven..” John 6:38
Christ is the visible expression of God. In His incarnation, Jesus is God in the flesh.