We know for a long time there was speculation about Limbo, and now recently the Church has said they have some theologians working on this topic, and the CCC says that we must trust in the mercy of God as relates to their salvation.
Here is how I came to a theory on what happens to ones like unborn babies - it occurred to me one day that everything God seems to do with man is based on “choice”. He didn’t just give man salvation, He wants us to use our free will to choose Him over evil. Adam and Eve had to choose. Mary had to choose, Israel had to choose, the angels had to choose, Moses had to choose (Pharaoh or God), the list goes on and on. Here’s another one – after Christ died on the cross, he descended into Sheol and preached the Gospel; presumably people there chose Christ (or not). So no one, angels or humans, gets to be with God without choosing Him in their heart and through their actions. It is the purpose of our time on earth.
So my theory is that what would happen with say an unborn infant is that somehow God will mature its soul to the point where it is able to to make a choice; to choose God, or not. It could be a test like Adam/Eve, or something different. However the opportunity for choice is presented, that is how the final disposition of their soul will be determined. I think there is a theological basis for a soul “maturing”, as I believe I have read that when a soul goes to Heaven it will grow in its understanding about God for the rest of eternity. Even if that is false, we certainly will grow in our understanding of God instantaneously once we arrive at Heaven, as we don’t have full revelation about God now. (So we are only unsure about the presence of time in the process, just like the process of purgatory).
When I think of how this might occur, it really sounds similar to what angels may have gone through - they had to choose God, or not, yet did not have the benefit of the beatific vision to influence their decision. They obviously had some basis on which to make their decision, we don’t know what that was (like we know the basis of Adam/Eve’s decision).
There is another argument supporting that we don’t completely “mature” immediately upon entrance to heaven, by the fact that we learn about all of mankind’s judgment on final judgment day at the end of the earth. That argues there are some things we don’t know while we are just a soul in heaven not united with our bodies.
So whether it takes all eternity or a fraction of time (or I could say whether this takes place within or outside of time), our soul has to “mature” in our understanding of who God is. The soul of an unborn is not restricted by anything of the body, so it is surely capable of the same maturation as the souls of older people who have had a chance to choose God on earth.
I just don’t see God giving anyone a free ride where they don’t have to use their free will to choose to love God, God doesn’t value that because it will be much less fulfilling to that person. Also, this theory seems theologically sound because it builds on/is consistent with principles revealed to us through the lives of the people in the Bible, the things we know about how people get to Heaven, and teachings like purgatory.
This is speculation of course - the only truth here is that Scripture does not reveal this to be true, nor does it reveal that it couldn’t be true. I just wanted to toss the idea to see what you think, since I have not seen this idea before.