The following is an excerpt from
The MacArthur New Testament Commentary on Hebrews 9.
And inasmuch as it is appointed for men to die once and after this comes judgment, so Christ also, having been offered once to bear the sins of many, shall appear a second time for salvation without reference to sin, to those who eagerly await Him. (Hebrews 9:27-28)
All men have to die, and our death is by divine appointment. It is one appointment everyone will keep. After death comes judgment, which is also appointed by God. And since men are not able to atone for their own sins, God's judgment demands that they pay or have a substitute pay for them.
Like all men, Jesus Christ was divinely appointed to die once. But unlike all other men, He will never face judgment. Because He took our sins upon Himself, He took our judgment upon Himself. But the judgment was for our sins, not for His, for He had none. God "made Him who knew no sin to be sin on our behalf, that we might become the righteousness of God in Him" (2 Cor. 5:21). He died the one death that judgment demanded.