I. CATHOLIC DOCTRINE

Purgatory (Lat., "purgare", to make clean, to purify) in accordance with Catholic teaching is a place or condition of temporal punishment for those who, departing this life in God's grace, are, not entirely free from venial faults, or have not fully paid the satisfaction due to their transgressions. The faith of the Church concerning purgatory is clearly expressed in the Decree of Union drawn up by the Council of Florence (Mansi, t. XXXI, col. 1031), and in the decree of the Council of Trent which (Sess. XXV) defined: "Whereas the Catholic Church, instructed by the Holy Ghost, has from the Sacred Scriptures and the ancient tradition of the Fathers taught in Councils and very recently in this Ecumenical synod (Sess. VI, cap. XXX; Sess. XXII cap.ii, iii) that there is a purgatory, and that the souls therein are helped by the suffrages of the faithful, but principally by the acceptable Sacrifice of the Altar; the Holy Synod enjoins on the Bishops that they diligently endeavor to have the sound doctrine of the Fathers in Councils regarding purgatory everywhere taught and preached, held and believed by the faithful" (Denzinger, "Enchiridon", 983). Further than this the definitions of the Church do not go, but the tradition of the Fathers and the Schoolmen must be consulted to explain the teachings of the councils, and to make clear the belief and the practices of the faithful.

Temporal Punishment

That temporal punishment is due to sin, even after the sin itself has been pardoned by God, is clearly the teaching of Scripture. God indeed brought man out of his first disobedience and gave him power to govern all things (Wis. x, 2), but still condemned him "to eat his bread in the sweat of his brow" until he returned unto dust. God forgave the incredulity of Moses and Aaron, but in punishment kept them from the "land of promise" (Num., xx, 12). The Lord took away the sin of David, but the life of the child was forfeited because David had made God's enemies blaspheme His Holy Name (II Kings, xii, 13, 14). In the New Testament as well as in the Old, almsgiving and fasting, and in general penitential acts are the real fruits of repentance (Matt., iii, 8; Luke, xvii, 3; iii, 3). The whole penitential system of the Church testifies that the voluntary assumption of penitential works has always been part of true repentance and the Council of Trent (Sess. XIV, can. xi) reminds the faithful that God does not always remit the whole punishment due to sin together with the guilt. God requires satisfaction, and will punish sin, and this doctrine involves as its necessary consequence a belief that the sinner failing to do penance in this life may be punished in another world, and so not be cast off eternally from God.

Venial Sins

All sins are not equal before God, nor dare anyone assert that the daily faults of human frailty will be punished with the same severity that is meted out to serious violation of God's law. On the other hand whosoever comes into God's presence must be perfectly perfectly pure for in the strictest sense His "eyes are too pure, to behold evil" (Hab., i, 13). For unrepented venial faults for the payment of temporal punishment due to sin at time of death, the Church has always taught the doctrine of purgatory.

So deep was this belief ingrained in our common humanity that it was accepted by the Jews, and in at least a shadowy way by the pagans, long before the coming of Christianity. ("Aeneid," VI, 735 sq.; Sophocles, "Antigone," 450 sq.).
http://www.newadvent.org/cathen/12575a.htm

Col 2:8-15
8 See to it that no one takes you captive through philosophy and empty deception, according to the tradition of men, according to the elementary principles of the world, rather than according to Christ. 9 For in Him all the fulness of Deity dwells in bodily form, 10 and in Him you have been made complete, and He is the head over all rule and authority; 11 and in Him you were also circumcised with a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ; 12 having been buried with Him in baptism, in which you were also raised up with Him through faith in the working of God, who raised Him from the dead. 13 And when you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, 14 having canceled out the certificate of debt consisting of decrees against us and which was hostile to us; and He has taken it out of the way, having nailed it to the cross. 15 When He had disarmed the rulers and authorities, He made a public display of them, having triumphed over them through Him. NAS