Romans 5: 12 Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned—
The text does not say that “all sinned in Adam”. Nor does this chapter ever use the word “imputation”. But the sin of verse 12 is not the result of death. The death is the result of “because all sinned”. So we look to the context to see how it is that the all sinned. They all sinned because of the representative sin of Adam.
Adam was our substitute. We don’t need to sin ourselves to be condemned to death. We are condemned to death because Adam sinned for us, as our representative. We are not guilty based on our corruption. Corruption is mediated to us because we are guilty. We sin but before that we were already constituted sinners.
Romans 5:13 “for sin indeed was in the world before the law was given, but sin is not counted where there is no law. 14 Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.”
It’s not only infants who died who did NOT sin like Adam. Everybody who died after Adam’s first sin but before the Mosaic law was given did NOT sin like Adam. Yet because of Adam’s sin and Adam’s representation, all these people died.
But didn’t they live a while and then die? Why did they get to live even a little if they were born guilty? Genesis: in the day you eat, you will surely die. The death is sure, but not yet executed upon all those represented by Adam.
“Jesus, remember me when you come into your kingdom.” “Truly I say to you
today you will be with me in Paradise.” Luke 23:43 Jesus did not wait until after He died to promise the justified thief that He would remember him. The promise to be with Jesus in paradise is sure, even on this day when Jesus died. “David foresaw and spoke about the resurrection of the Christ, that he was not abandoned to Hades, nor did his flesh see corruption. This Jesus God raised up.” Acts 2:31
Romans 5:16 “For the judgment following one trespass brought condemnation, but the free gift following many trespasses brought justification. 17 For if, because of one man’s trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ.”
God has a just nature but the Bible never says that death for Adam’s guilt imputed in any way undermines the goodness of God
Imputation is not only about the punishment of sins. Imputation is about the bearing of guilt. Infants who die have been imputed with Adam’s sin, and that doesn’t mean they only bear the punishment of Adam’s sin. They sinned when Adam sinned.
So how did the elect die when Christ died? Again, the logical inference is by imputation of what their representative accomplished. What did Christ get done? What did Christ finish? Christ died for the sins of the elect imputed to Him. Not only did God impute the sins of the elect to Christ, but God also imputes the death of Christ (this good and just death, because of sins) to the elect, in time and individually, one by one as God justifies these elect.