It would not be right or fair for God to just forgive everyone. There is a penalty that must be paid for doing wrong. The penalty for sinning against God is death, eternal death. But God loved us so much that he sent Jesus, his only Son, to pay our penalty. Jesus did this by dying on the cross, in our place. Now everyone can be forgiven by trusting in Christ.
Yes, all have sinned. All fall short of God's perfect glory. But if we trust in Jesus Christ, God says we are "not guilty." In his kindness he freely takes away our sins. God sent Christ Jesus to take the punishment for our sins. He ended all God's anger against us. Christ's blood and our faith saves us from God's anger. This is the way he could be fair to everybody. He did not punish the people who sinned before Christ came. He was looking forward to the time when Christ would come and take away those sins. (Romans 3:23-25)
Mark 8:31; John 3:16; Romans 5:8; Hebrews 2:14-17; 8:3; 9:13-14, 22-23; 1 John 1:7