Of course, the paradise the Lord Jesus promised the one thief meant heaven: a distinct place beyond our cosmos filled with the presence of the Lord and all His holy angels. And though with our earthly brains and experiences, heaven is incomprehensible, that certainly does not lessen its reality.
At the same time, eternal Life (seemingly used in Scripture interchangeably with the idea of heaven) begins at the moment of salvation. The one thief passed from death unto eternal Life while on he was on the cross. And His Life continued through his physical death on the cross.
For me this reinforces the Believer’s understanding of the truth of eternal security.
If, as been suggested earlier, when the Lord Jesus said “today you will be with me in paradise,” and today could be interpreted as meaning, “at this very moment;” when our Lord gives eternal Life to a needy soul, it is unthinkable that He would, at some later moment, take back this gift of eternal Life which has already been part of his present reality.