Devine Providence has led you to ask this, at this time.
Just moments ago I responded to a fallen-away catholic, who wrote a post on his Blog [AS A FORMER Catholic no less], claiming it is not true; and just another lie taught by the CC.
So I’ll offer you a choice of the short account, or one more detailed and better fleshed out. The Short account here, and the longer one as an attachment. Also I’m working on a more complex discussion that will take me awhile to complete. It’s not at the top of the stack of stuff to get done, so I’m not sure when it will be finished?
The basis of his claim lies in the fact that the term is not in the bible and therefore MUST be a[nother] Catholic invention. So let’s begin at that point.
Let’s look at a few other terms that don’t appear in the bible:
And even the term “bible” are not found in Holy Writ. So that reason is a “lame-excuse.”
Each of these terms has a biblical foundation, even if by implication. But the existence of Purgatory does have a solid biblical foundation, as will shown.
So why then do Protestants “harp” on this topic? Because it’s sellable! Because it’s not in the bible specifically, they seem to have some credibility on the issue. More so than claiming we Catholics “WORSHIP” Mary; which is an even more egregious error on their part.
Because there is BUT One God; who logically can have ONLY One set of faith believes, and who choose only One Chosen people in the OT … Leviticus 26:12 “I will walk among you, and will be your God, and you shall be my people” & One Church in the NT “Matthew 16: 18-19 “And I say to thee: That thou art Peter; and upon this rock [YOU; Peter] I will build my church, [Singular] and the gates of hell shall not prevail against it.[Again Singular]And I will give to thee [ALL of] the keys of the kingdom of heaven. And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose upon earth, it shall be loosed also in heaven.” Because God blocks right and full understanding of all other faiths and churches as each of them have chosen to set themselves up in opposition to God; to what God desires, and what God choose to do, and clearly so; they are penalized with an inability to understand Until they repent and actually turn back towards the Lord. These terms “binding and loosening” are terms of unlimited Powers and authority of Governance. Thus the church is empowered to create theological terms and definitions. John 17: 18 & 20:21 prove this quite clearly.
Protestants do this out of self-interest and absolute need to justify setting themselves up in opposition to the ONLY faith & church founded by Christ. In order to justify their existence they MUST attack the Catholic Church and Her beliefs. Because what we hold IS God’s truth; they must reinvent the truth to support there positions. That’s enough background.
One final observation. In addition to not being ABLE to rightly understand the bible; they lack an ability to have an informed understanding of the Devine Nature of God as well. Thus they invent the Sinners prayer, OSAS, Salvation by Christ doing ALL the Work, and so on. None of these things being even close to what God Commands for salvation. Some even deny the necessity of baptism. John 3: 5.
So Purgatory not being understood flows directly from their inability to understand God; and  God’s Bible, and the nature of sin itself.
PURGATORY. “The place or condition in which the souls of the just are purified after death and before they can enter heaven. They may be purified of the guilt of their venial sins, as in this life, by an act of contrition deriving from charity and performed with the help of grace. This sorrow does not, however, affect the punishment for sins, because in the next world there is no longer any possibility of merit. The souls are certainly purified by atoning for the temporal punishments due to sin by their willing acceptance of suffering imposed by God. The sufferings in purgatory are not the same for all, but proportioned to each person’s degree of sinfulness. Moreover, these sufferings can be lessened in duration and intensity through the prayers and good works of the faithful on earth. Nor are the pains incompatible with great peace and joy, since the poor souls deeply love God and are sure they will reach heaven. As members of the Church Suffering, the souls in purgatory can intercede for the persons on earth, who are therefore encouraged to invoke their aid. Purgatory will not continue after the general judgment, but its duration for any particular soul continues until it is free from all guilt and punishment. Immediately on purification the soul is assumed into heaven. (Etym. Latin purgatio, cleansing, purifying.)”
 Not understanding God, they fail to recognize that God’s Desire to be in a Personal Relationship with man, who alone can choose to [OR NOT TO], Know, Love, serve, Obey and Worship Him [Isaiah 43: verses 7 & 21] so they miss the absolutely from a Divine Justice perspective, Gods desire and NEED, to save as many souls as is possible.
Purgatory makes possible for Millions of souls [perhaps even Billions?] that otherwise are not perfect; but “only very good.”[Dies without unconfessed or unforiven Mortal sins] ; which misses the mark of being perfect; BECAUSE God is perfect he can accept nothing less than a Perfect [or perfected] Soul in His Presence. Matt.5: 4” You, therefore, must be perfect, as your heavenly Father is perfect”  Then they miss Purgatories necessity by overlooking [blind presumption on their part ]that all sin has a second nature, what the Church terms a “public nature” /Temporal Punishment,” because all sin hurts others besides the sinner. God seeking Perfect Souls insist that this debt be repaid in full prior to entry into heaven, and this debt exist even AFTER [and even IF] the sin has been sacramentally forgiven [John 20:19-23], which they also deny is necessary.
This is a bit complex, but the amount due, as well as our attempts to repay it while still on earth are impeded because Christ alone knows the amount of the debt, and the amount of credit He will accept and apply to this debt by our prayers, charity, and Indulgences. So the Church exercising her Authority [John 17: 18 & john 20:21, Matthew 16:18-19] offers us the opportunity to MERIT
Plenary Indulgences [ a separate topic and Lesson], that have the Power to wipe clean the debt completely until we sin again.
The last issue is the Bible itself. Here is the short form, the attachment has much more.
Lev.22: 21 “And when any one offers a sacrifice of peace offerings to the LORD, to fulfill a vow or as a freewill offering, from the herd or from the flock, to be accepted it must be perfect; there shall be no blemish in it.
Rev. 21: 27 “But nothing unclean shall enter it, nor anyone who practices abomination or falsehood, but only those who are written in the Lamb’s book of life.”
Mt. 5: 26 truly, I say to you, you will never get out till you have paid the last penny.
Matt.5: 48 “You, therefore, must be perfect, as your heavenly Father is perfect.”
Heb. 2: 10 For it was fitting that he, for whom and by whom all things exist, in bringing many sons to glory, should make the pioneer of their salvation perfect through suffering.
1 Cor. 3: 13-14 “each man’s work will become manifest; for the Day will disclose it, because it will be revealed with fire, and the fire will test what sort of work each one has done if the work which any man has built on the foundation survives, he will receive a reward. If any man’s work is burned up, he will suffer loss, [Purgatory] though he himself will be saved, but only as through fire.
So that is the short version. Please let me know if you still have questions. I suggest that we might think of Purgatory as a Sort of “Soul-Wash” [Longer and more painful], but accomplishing making the soul “Perfect” for entry into the Beatific Vision.
God Bless you,