This is what I tell the kids: You did something wrong. I forgive you. You still have to pay a consequence for what you did. That doesn't change my love for you, but it makes you realize that what you did was wrong and something must be done to make up for the offense. It perfects you. I don't like to see you have to endure the consequence, but I love you enough to want you to be the best that you can be.
If this parenting technique, employed to help children understand that choices have consequences, is reasonable, why would we expect any less from Our Heavenly Father in dealing with us? Heaven is where we are meant to be with the Trinity eternally enjoying the Beatific Vision. But, Scripture, Doctrine and Tradition are in harmony on this point: One must be perfected to enter heaven.
I found a website, Scripture Catholic, that lays it all out clearly (I won't try to duplicate it here -- they do an excellent and thorough job on the site). It draws from Scripture, Tradition and the Fathers of the Church. It is really worth a look, especially if you know people who don't believe in this sojourn of perfection after death. But, I'd like to add one more dimension -- the Catechism of the Catholic Church.
The Catechism says this about purgatory:
III. THE FINAL PURIFICATION, OR PURGATORY
1030 All who die in God's grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven.
1031 The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned. The Church formulated her doctrine of faith on Purgatory especially at the Councils of Florence and Trent. The tradition of the Church, by reference to certain texts of Scripture, speaks of a cleansing fire:
As for certain lesser faults, we must believe that, before the Final Judgment, there is a purifying fire. He who is truth says that whoever utters blasphemy against the Holy Spirit will be pardoned neither in this age nor in the age to come. From this sentence we understand that certain offenses can be forgiven in this age, but certain others in the age to come.1032 This teaching is also based on the practice of prayer for the dead, already mentioned in Sacred Scripture: "Therefore [Judas Maccabeus] made atonement for the dead, that they might be delivered from their sin." From the beginning the Church has honored the memory of the dead and offered prayers in suffrage for them, above all the Eucharistic sacrifice, so that, thus purified, they may attain the beatific vision of God. The Church also commends almsgiving, indulgences, and works of penance undertaken on behalf of the dead:
Let us help and commemorate them. If Job's sons were purified by their father's sacrifice, why would we doubt that our offerings for the dead bring them some consolation? Let us not hesitate to help those who have died and to offer our prayers for them.
In these four short paragraphs the Church sums up its catechesis on Purgatory. There is a temporary stage in which the soul is purified and endures sufferings with assurance and hope of the eternal life to come.
Holiness is not an easy path to walk. No. It is a narrow gate through which a soul must enter into heaven. (cf. Mt 7:14), And God gives man every opportunity to muster his resolve when he falls so that he might try again. Mercy changes everything. The Sacrament of Penance is critical to staying on the right path -- a well formed conscience that intervenes and reminds us that choices have consequences, and due reparation attached, keep that path unfettered and passable.
Pray for the dead with the hope of heaven on this All Souls Day. All the souls in purgatory will return to you in gratitude their intercession from heaven!