If I ran into someone who didn't believe that Paul belonged in the canon for whatever reason (a modern Ebionite), I could still discuss theology with him and present my theological views from the other 53 books that he does accept.
If I ran into a member of the Syrian church, I could still present a pre-millennial eschatology to him without once referring to the book of Revelation.
When I talk to a Jew, I can present the Messiah to him without needing to convince him of the canonicity of the New Testament.
Can you?
Likewise, can you prove that we're supposed to pray for the dead because there is supposedly a place called Purgatory without having to refer to a disputed book or the dictates of Roman Catholic tradition, which I no more accept as authoritative than I do the Talmud?
If you think you can, give it a shot. If not, you may just have a problem.
I just did, or was I too cryptic?
The unmerciful debtor is condemned for his debt to slavery. This is eternal punishment of hell.
He pleads for mercy and receives it. His debt is forgiven him. This is the atonement of Christ available to the believers.
He then does not live out the life of mercy himself. This is a Christian who sins. But the Lord does not revoke the forgiveness already given. He imposes a temporal punishment rather than an eternal punishment. He is sent to jail, -- not to slavery, -- until he pays off the last penny. This is Purgatory.
Now, why pray for the souls in Purgatory? Because men asked Christ to return to life their dead loved ones, and He responded. Raising of Lazarus and of Jairus's daughter is seen as giving eternal life in Paradise facing Christ. So, we can petition Christ for the dead and He will raise them from the dead, -- release them from Purgatory to Heaven.
You may, and probably will, offer a different interpretation of the parable and of the raising of the dead episodes, but that is not the point, as the Catholic interpretation is consistent with the Scripture and, of course, with the patristic tradition.