Limbo

I. The Limbo of the Fathers - A place and state of rest wherein the souls of the just who died before Christ's ascension were detained until he opened Heaven to them; referred to as "Abraham's Bosom" (Luke xvi,22) and "Paradise" (Luke xxiii, 43) and notably in Eph. IV, 9 and I Peter iii, 18-20.
II. The Limbo of Children - It is of faith that all, children and adults, who leave this world without the Baptism of water, blood or desire and therefore in original sin are excluded from the Vision of God in Heaven. The great majority of theologians teach that such children and unbaptized adults free from grievous actual sin, enjoy eternally a state of perfect natural happiness, knowing and loving God by use of their natural powers. This place and state is commonly called Limbo.
(Definition from A Catholic Dictionary, 1951)
 

References in Scripture:

• "And it came to pass, that the beggar died, and was carried by the angels into Abraham's bosom. And the rich man also died: and he was buried in hell" Luke 16:22
• "Now that he ascended, what is it, but because he also descended first into the lower parts of the earth" Ephesians 4:9
• "Because Christ also died once for our sins, the just for the unjust: that he might offer us to God, being put to death indeed in the flesh, but enlivened in the spirit, In which also coming he preached to those spirits that were in prison: Which had been some time incredulous, when they waited for the patience of God in the days of Noe, when the ark was a building: wherein a few, that is, eight souls, were saved by water." 1 Peter 3:18-20
• "And he said to Jesus: Lord, remember me when thou shalt come into thy kingdom. And Jesus said to him: Amen I say to thee, this day thou shalt be with me in paradise" Luke 23:42-43
   

Church Teaching:

• "Moreover as Christ was true and perfect man, He of course was capable of dying. Now man dies when the soul is separated from the body. When, therefore, we say that Jesus died, we mean that His soul was disunited from His body. We do not admit, however, that the Divinity was separated from His body. On the contrary, we firmly believe and profess that when His soul was dissociated from His body, His Divinity continued always united both to His body in the sepulchre and to His soul in limbo. It became the Son of God to die, that, through death, he might destroy him who had the empire of death that is the devil, and might deliver them, who through the fear of death were all their lifetime subject to servitude." Catechism of Council of Trent, The Creed, Article IV
• "Q: What are we taught in the Fifth Article: He descended into hell; the third day He rose again from the dead?
  A: The Fifth Article of the Creed teaches us that the Soul of Jesus Christ, on being separated from His Body, descended to the Limbo of the holy Fathers, and that on the third day it became united once more to His Body, never to be parted from it again" Catechism of St. Pope Pius X, The Fifth Article of the Creed
• "Q: What is here meant by hell? A: Hell here means the Limbo of the holy Fathers, that is, the place where the souls of the just were detained, in expectation of redemption through Jesus Christ" Catechism of St. Pope Pius X, The Fifth Article of the Creed
• "On the contrary, we firmly believe and profess that when His soul was dissociated from His body, His Divinity continued always united both to His body in the sepulchre and to His soul in limbo" ("Roman Catechism," Fourth Article, 6)." Catechism of St. Thomas Aquinas, The Creed, The Fourth Article, End Notes
• "The fourth and final reason is that Christ might free the just who were in hell [or Limbo]. For as Christ wished to suffer death to deliver the living from death, so also He would descend into hell to deliver those who were there". Also, "The reason they were there in hell [i.e., Limbo] is original sin which they had contracted from Adam, and from which as members of the human race they could not be delivered except by Christ. Catechism of St. Thomas Aquinas, The Creed, The Fifth Article, Reasons for Christ's Descent
• "Therefore, "He descended into hell" means that the soul of Jesus Christ, after His death, descended into Limbo, i.e., to the place where the souls of the just who died before Christ were detained, and were waiting for the time of their redemption" Catechism of St. Thomas Aquinas, The Creed, The Fifth Article, End Notes
• "The limbo of the Fathers and the limbo of children, without any doubt, differ as to the quality of punishment or reward. For children have no hope of the blessed life, as the Fathers in limbo had, in whom, moreover, shone forth the light of faith and grace. But as regards their situation, there is reason to believe that the place of both is the same; except that the limbo of the Fathers is placed higher than the limbo of children, just as we have stated in reference to limbo and hell." Summa Theologica, St. Thomas Aquinas, Whether the limbo of children is the same as the limbo of the Fathers?
• "Suarez, for example, ignoring Bellarmine's protest, continued to teach what Catharinus had taught -- that unbaptized children will not only enjoy perfect natural happiness, but that they will rise with immortal bodies at the last day and have the renovated earth for their happy abode (De vit. et penat., ix, sect. vi, n. 4); and, without insisting on such details, the great majority of Catholic theologians have continued to maintain the general doctrine that the children's limbo is a state of perfect natural happiness, just the same as it would have been if God had not established the present supernatural order" 1917 Catholic Encyclopedia, Limbo
• "Thus the Council of Florence, however literally interpreted, does not deny the possibility of perfect subjective happiness for those dying in original sin, and this is all that is needed from the dogmatic viewpoint to justify the prevailing Catholic notion of the children's limbo, while form the standpoint of reason, as St. Gregory of Nazianzus pointed out long ago, no harsher view can be reconciled with a worthy concept of God's justice and other attributes." 1917 Catholic Encyclopedia, Limbo
   

Summary

Contrary to what some Catholics have come to believe today, the doctrine of Limbo is mentioned in Scripture (albeit by a different name) and as we can see above, has been taught century to century by the Catholic Church. To deny its existence is not Catholic.