I had (and still do) have a hard time reconciling the Church’s stance on eliminating the concept of Limbo. Although it came as a total surprise to me (and to the vast majority I presume) that Limbo was never an official Church teaching but a theological construct, I still feel that it is still a valid explanation as to what happens to those good souls who have never been baptized.
Catholicism teaches that every newborn babe born has the effects of original sin on his/her soul, and that baptism, as a “sacred act” (sacrament) washes away this effect and imbues the soul with supernatural grace. Thus, the individual now becomes part of the Church community and his/her journey towards sanctification has begun.
So, if a newborn does not have this opportunity, what happens to his/her soul if he/she dies? Medieval theologians believed that they might exist in a ‘limbo’ state, a state of happiness but without the Beatific Vision. In that sense, they are never condemned to Hell but do not attain Heaven either. And what happens on the “last day” when bodies are finally united with their souls? Hopefully, they will eternally be in Heaven as all other "in-between’ states will no longer exist.