because the Bible tells us that He is. For instance,
John 1
1 In the beginning was the Word, and the Word was ~with~ God, and the Word ~was~ God.
2 He was in the beginning with God.
3 All things came into being by Him, and apart from Him nothing came into being that has come into being.
4 In Him was life, and the life was the Light of men.
14 And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth.
As for why it was necessary for Him to come here as our Savior (not simply as our guide), and why both the Incarnation and the Cross were absolutely necessary for our salvation, here are some Scripture verses that should help begin to explain that.
Romans 5
8 God demonstrates His own love toward us, in that while we were yet sinners, Christ ~died~ for us.
9 Much more then, having now been justified by His blood, we shall be saved from the wrath of God through Him.
10 For if while we were enemies we were reconciled to God through the death of His Son, much more, having been reconciled, we shall be saved by His life.
2 Corinthians 5
21 He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him.
Hebrews 9
22 All things are cleansed with blood, and without shedding of blood there is no forgiveness.
This seems like plenty for now, so I'll stop here to see if you have any comments or questions, and to allow some other members to weigh in.
God bless you!!
--Deuteronomy
Isaiah 53
5 He was pierced through for our transgressions,
He was crushed for our iniquities;
The chastening for our well-being fell upon Him,
And by His scourging we are healed.
6 All of us like sheep have gone astray,
Each of us has turned to his own way;
But the LORD has caused the iniquity of us all
To fall on Him.
1 Peter 2
24 He Himself bore our sins in His body on the cross, so that we might die to sin and live to righteousness; for by His wounds you were healed.