The need for the cross

God is all knowing, always loving, always present, always just and always holy.

God is always present, you cannot hide your sin from Him, He was there when you committed it. 
God is all knowing. You cannot hide your sinful thoughts and desires from Him.
God is always just, He may delay His judgment, but He cannot allow sin to go unpunished. 
God is always holy, He cannot have fellowship with sin. 
God is always loving, He desires to have fellowship with humanity.

The attributes of God are many and cannot be compromised.  God has no beginning and no end.  He created the entire universe and controls everything in it.  He is the source of life and He defeated death.  If any of His many attributes were to fail or be compromised He would not be God.
 ""I can see what they are doing, and I know what they are thinking. So I will gather all nations and peoples together, and they will see my glory." {Isaiah 66:18 NLT}
 When God created He desired to have fellowship with His creation, He wants to be with us and dwell among us.  But God wants us to choose Him, He does not force Himself upon us but asks for us to freely choose Him.

"Jesus replied, "All who love me will do what I say. My Father will love them, and we will come and make our home with each of them. Anyone who doesn't love me will not obey me. And remember, my words are not my own. What I am telling you is from the Father who sent me." {John 14:23-24 NLT}
 But for those who do choose to serve the God of the Bible the problem of sin remains. Everyone has sin, there is no one who is holy (Romans 3:23). God cannot have fellowship with someone who is in sin and spiritual darkness (1 John 1:6). Sin cannot simply be forgiven, it’s too terrible a crime in the eyes of God, and must always be punished with death to satisfy the justice of God (Hebrews 9:22).  But if we die in our sins as punishment for our sins we are forever separated from God (revelation 21:8).

"The disciples were astounded. "Then who in the world can be saved?" they asked. Jesus looked at them intently and said, "Humanly speaking, it is impossible. But with God everything is possible."" {Matthew 19:25-26 NLT}
 God found a way to satisfy both His justice and His love, that answer is the cross.

God came to earth as the man Jesus.  (Philippians 2:6-7).  Jesus lived a sinless life and sacrificed himself on the cross to take the punishment for our sins, making the forgiveness of our sins possible.  (2 Corinthians 5:21).  Jesus rose from the dead to bring life after death to all those who follow Him (2 Corinthians 4:14).

Consider it this way, we stand guilty before the judge, the sentence is death by hanging.  There is no possibility of parole or appeal.  As the gavel sounds and the weight of your fate falls upon you the judge stands up and orders the bailiff to take himself into custody.  As the judge hangs for your crime you are set free, the punishment for your crime has been paid by another.  You are now declared innocent under the law, your debt has been paid.

We can be forgiven only because God Himself, in the person of Jesus, took our punishment on the cross.  We can have new life and fellowship with God because after Jesus carried away our sin in death He rose to new life so that we also can live with Him after death.

Thanks to the cross we now have a choice, we can choose to pay the penalty for our own sins and experience an eternity of God’s wrath in Hell or we can accept the free gift of life by accepting that Jesus died on the cross for our sins and place our faith and obedience in Him so that we may live eternally with God.

"And as Moses lifted up the bronze snake on a pole in the wilderness, so the Son of Man must be lifted up, so that everyone who believes in him will have eternal life. "For God loved the world so much that he gave his one and only Son, so that everyone who believes in him will not perish but have eternal life. God sent his Son into the world not to judge the world, but to save the world through him. "There is no judgment against anyone who believes in him. But anyone who does not believe in him has already been judged for not believing in God's one and only Son." {John 3:14-18 NLT}