What really is the penalty for sin?

The wages of sin is death (Romans 6:23).

And death is death -- it is either Christ's death, or your own death that will atone for your sin.

For the joy that was before him, Jesus endured the cross; He despised the shame, and He is now seated at the right hand of the throne of God. He is the author and finisher of our faith (Hebrews 12:2). Jesus' death brought joy. We can confidently trust Him.

Hebrews 12:2
Looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despised the shame, and is set down at the right hand of the throne of God.

Christ is divine. For our salvation He became man. As man He could die. By His death He paid the penalty for our sin. Through His divinity He rose again; for life is inherent in Him. (John 10:17, 18) In victory, Christ paid our debt, He set us free, He gave us everlasting life (John 3:16).

In contrast, we are mortal; if we die for our own sin we will never live again (2 Peter 3:7, 13; Revelation 20:14, 15).