My limited understanding is this:
At baptism, we are all given Sanctifying grace. This is what enables us to even begin to be elegible for eternal salvation. Sanctifying grace is killed in us by mortal sin, and can only be restored through the Sacrament of Confession.
Through our actions, our prayers, attending Mass, and, above all, God’s own boundless generosity, we are also given Actual grace, which helps us grow in holiness.
Each soul, due to free will, will respond differently to different situations. Say, for example, two baptised people are both faced with the opportunity to commit “the perfect crime”. Person A, though baptised, hasn’t taken advantage of the graces poured out by God, and so has conditioned his free will to act in a selfish, immoral manner. Person B has taken advantage of the undeserved gift of God’s graces, and so has conditioned his soul to act in a completely different manner.
Or, to use a different metaphor: two people, initially identical in physical attributes, both decide to run a marathon. Person A runs maybe a few times a week, but mostly puts off training and sits around and eats a lot of fast food. Person B finds and follows a training schedule, eats nutritious foods, and gets plenty of rest. They started out in the same condition, but, due to their freely chosen actions, ended up with wildly different results.
So it is with our souls.