Purgatory is an intermediate state of purification between death and heaven for those who die with small sins for an amount of time appropriate to the amount and severity of the sins as deemed by God to remove the temporal effects of sin. It is the final purification so that one’s soul can enter heaven unblemished.
Those who make it to purgatory, die in the state of grace. They are the children of God, who before death, were guilty of venial sins or have failed to do sufficient penance for sins already forgiven. Purgatory is not an opportunity where the unrepentant can avoid hell.
In Revelation 21:27, God tells us that nothing imperfect can enter heaven.
“but nothing unclean will enter it, nor any[one] who does abominable things or tells lies.”
Only those with no blemishes on their souls immediately enter heaven. Those who have not repented and confessed their sins will go to hell. Seeing as how, when we die, many of us will not fit in either of those two extreme categories we must fit somewhere else, somewhere in the middle called purgatory.
The majority of people are neither so free from sin as to merit immediate entrance into heaven, nor so bad as to be punished forever in hell.
Those people who have repented and confessed their sins will likely go to purgatory on their way to heaven to be purged removing the temporal effects of sin so that the soul is clean enough to enter heaven. One can reduce the amount of time spent in purgatory by means of an indulgence.
Purgatory is rooted in the Bible. Assurance of salvation and automatic entrance into heaven are not biblical. For if that were true one would have to be as perfect as Jesus, of which no human is capable; you would have to not have the ability to sin.
Everyone is a Sinner
We know that God gave each of us the freedom to sin by giving us free will. Sin is the rejection of God and we can either choose God or reject God even after baptism. Thankfully God offers us the sacrament of Penance and Reconciliation to make right again our relationship with him and purgatory to cleanse ourselves to be in perfect union with him in heaven.
At heart every Christian believes in purgatory, no matter how vehemently one might denounce it by word. This is evident from the prayers said almost unconsciously for deceased friends and relatives. There is scarcely a Christian funeral without prayer; it is a case of actions speaking louder than words. If there is no such thing as purgatory then what is the point of prayer for the dead? If a soul is in heaven it does not need prayer; if it is in hell, prayer cannot help it.
Purgatory is Rooted in God’s Justice
Human justice and divine justice are somewhat similar because our human system is modeled after the divine system. Without the divine justice system we could have no human justice system.
Human justice recognizes big criminals and little criminals and punishes them accordingly. It has a jail to punish some criminals for one, ten, or thirty days, and a prison where it punishes other criminals for several years to lifetimes.
For instance, pretend that in your city two people are arrested, one for speeding and one for murder, and that both people are sentenced to prison for life. Do you consider it just when one who speeds commits a lesser crime than the other who murdered, yet the speeder receives the same punishment as the murderer? Lesser crimes deserve lesser punishment. Denying purgatory is to accuse God of dealing unjustly with his disobedient creatures.