Note that cumulative charity through incarnations is simply a Western, and Christian, interpretation of karma. Every life is then a dangerous adventure in which one grow in charity through righteousness and grace, or lose charity through wicked thoughts, words, or deeds.
The singular importance of this theological virtue is noted by St. Thomas: “Hence he who possesses the more charity, will see God the more perfectly, and will be the more beatified.” (ST: I, 12, 6)
As I mention in my Secrets of Metaethics, there are three kinds of light: 1st-level physical light, 2nd-level spiritual light of the chakras, and 3rd-level divine light. It belongs to the soul on the 2nd level to, through grace, acquire the 3rd-level divine light. (Though it is more accurate to say that this light acquires you.) Faith is such light in the intellect, charity in the will, and hope in power.
So to summarize, the lower 5 chakras develop naturally during pregnancy; the intellect which is the higher 2 chakras descends from heaven and completes the soul some time around quickening, though it is wiped clean and loses all memories upon that event; charity accumulates in the heart from one incarnation to the next. Of course, a little child cannot be said to have charity understood as explicit rational friendship with God. So we must contend that the holy light exists in the will at first in a latent form and later bursts forth, igniting at the appropriate time in the right conditions.
It is hard to see why Christianity has denied a doctrine so convenient.
Indeed, reincarnation solves a number of perplexing problems. One is no longer forced to postulate either the Limbo of the Fathers or of the Children which are little more than desperate expedients. Neither Abraham nor Socrates were hopeless cases. Fully ensouled babies who are aborted in heaven recover their intellects and face neither increase nor diminution of their charity.
There are three not two states in life: corrupted nature, pure nature, and grace. (The fourth state, glory, is in heaven only.) In Christianity, the wicked may well suffer serious punishment, the graced will see God, but what happens to the naturally righteous of whom there have been billions? It is madness to consign them to hell, indeed no better doctrine for making atheists exists. So will they enjoy natural happiness without the vision of God? But that’s not God’s MO at all. He does not need mere humans, or even angels, He wants sons who share to some extent His 3rd-grade nature (that is, consisting of 3 levels). Reincarnation allows these people to try again.
It grants greater mercy to the wicked. Even if you lost some charity due to sin in one life, you may be able to recover it in another.
It makes more sense of the distinction between Immediate Judgement after death and Last Judgment at the end of the world. One distinction is that the latter includes all the remote consequences of one’s actions in a single life. But it is unclear how that can affect the final happiness if one is in no way responsible for what happens thousands of years after his death and that any influence one might have had on the future is intertwined with countless other factors. Reincarnation gives rise to a more significant distinction which is that Last Judgment is one that tallies up all your lives.
I’m sure this list can be extended. Reincarnation seems like the height of common sense to me.