Limbo
The term is used in two contexts: the place where the souls of the just men of the Old Covenant (e.g. Adam, Abraham, Moses, etc.) abode after they died before Christ's Crucifixion made satisfaction for the Fall (called limbus patrum, limbo of the fathers); and the place where the souls of those who have not attained the age of reason, who die without being baptized (called limbus infantium, children's limbo). Both play a key rôle in Catholic theology with regards to Baptism and sin.
Limbus patrum was clearly necessary, since Heaven was closed to man after the Fall and could never merit it of himself. However, it would not be according to God's justice for all those who followed the commandments of God - either implicitly before they were solemnly proclaimed on Mount Sinai, or explicitly thereafter - and expected the promised Redeemer, to be condemned to Hell. St. Paul explicitly tells us that Abraham was justified before God, and it would be unreasonable to think that many others were not as well. Hence, there must have been a neutral place were their souls dwelt until Christ could proclaim their redemption. It is alluded to several times in the New Testament (as in Our Lord's parable of Lazarus and Dives) and explicitly so by St. Peter (I Peter 3:18-20). It is bound up in that wondrous verse from the Te Deum, "Tu devicto mortis aculeo: aperuisti credentibus regna caelorum", "Thou, having overcome the sting of death: didst open the kingdom of heaven to those who believe". It is explicit in the Apostles' and Athanasian Creeds, "Descendit ad inferos", "He descended into hell". But the Reformers, particularly Calvin, could not accept it, firstly because it did not accord with their view of God's justice as being merely vengeful and not merciful also, secondly because the admittance of one intermediate place between Heaven and Hell opened the possibility of another, that is Purgatory.
Once the Sacrament of Baptism was available, however, such a state was no longer required: whoever received it was freed from Original Sin and, if they thereafter died without stain of mortal sin, were assured of the beatific vision; those who died having knowingly rejected it would be punished in Hell (I will not speak of those who die, having attained reason, without having the opportunity to receive it - that is something of a theological minefield which I am not qualified to negotiate).
But what of those who, not having attained reason and therefore being incapable of committing sin, die without receiving Baptism? They cannot enter Heaven for the same reason as Abraham et al. could not: they are still stained with Original Sin. However, it would not be just for them to be punished in Hell since they have never done anything of themselves to deserve it. The early Fathers therefore said that their souls where in some state that was free from positive punishment (as in Hell), but were deprived of the happiness of God (in Heaven), some even allowing that they enjoy some sort of 'natural' happiness. Augustine (in his zealous refutation of Pelagius, a proto-sola fideist) denied this, saying that those souls participated in the punishment of the damned, allowing that it is perhaps very slight. Since then, the Doctors of the Church have gradually returned to the pre-Augustinian view.
It is worth noting that the nature of limbus infantium is speculation: the Church has no clear revelation on this matter. Its existence is, however, required by our understanding of the necessity of Baptism and God's justice. Needless to say, limbus patrum is not up for negotiation as it is clearly referenced in Scripture and the Creeds.
It is odd to think that many people's idea of Limbo as some featureless void with babies floating around in it. I prefer the idea that they are in some place where, ignorant of God since they have never had the ability to know him, they exist in happiness, as far as one can without God: blissful ignorance, if you like.
I hope that your Lenten disciplines are going well, and that they are leading you into a deeper and closer knowledge of Our Lord.
Gratia Dei et intercessione Sanctae Genetricis suae, sim semper frater dilectus tuus in Christo,
Joannes
