God exists as three interdependent persons (God the Father, Jesus the Son, and the Holy Spirit) whose essence are one and the same.
Jesus Christ, who is God - the divine being, laid aside His divine rights and privileges to become a human being. He did not give up His divinity, rather He accepted self-imposed limitations by becoming a human man, yet without sin. This He did to become qualified as humanity's representative before the God the Father.
Humanity is God's creation and was good in His eyes. However, when the first created man (Adam) chose his own will above God's, that man fell from his sinless state taking all of humanity with him as a result. Mankind became sinful by nature as Adam's spiritual DNA has passed to all of humanity.
God's nature is holy and the nature of humanity is unholy (sinful), therefore, God could not continue in fellowship with humanity.
Because God is loving and merciful, God the Father sent His Son Jesus to become humanity's perfect representative so that He might redeem fallen mankind and restore the fellowship for which humanity was first created. And this Jesus accomplished on the cross by willingly offering His life as the sacrificial payment for the sins of all. His resurrection proves both that He was who He claimed to be (God in the flesh) and that His sacrifice was accepted by the God the Father. As such, any person who believes these things shall be made a child of God and been granted mercy, forgiveness, and everlasting life.