What was the reason for the cross?
I can answer your friend’s question in one word sin. Life is short. Death is sure. Sin the curse, Christ the cure. And how does He cure? Through the cross of the Lord and Savior Jesus Christ.
First Peter 3:18 says, “For Christ also hath once suffered for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh, but quickened by the Spirit.”
Suppose someone punched you in the nose. And suppose in an act of compassion, you were to say to that person, “I forgive you.” And suppose they said to you, “Oh, there’s no need for you to forgive me. I’ve already forgiven myself.” And then, suppose the person standing by said, “Well neither one of you need to worry about it. I have forgiven both of you.”
That’s not how forgiveness works. Only the one who was punched can forgive the puncher. Sin is a clenched fist in the face of God and only God can forgive sin.
Away with all of the psycho-babble that says we need to forgive ourselves and affirm one another. God is a holy God and sin is a rebellion against a holy God. And Jesus dealt with our sin with His blood. And now we are reconciled to God.
In a very brief answer, that is why there was a cross. We were adrift on a sea with no hope of being rescued. And God devised a divine plan that His banished will not be expelled from Him. God found a way of bringing us back and it was through the cross.