We can understand why a Calvinist would find the existence of purgatory difficult to accept. You are either a saint on the way to heaven or you're damned and destined for hell.However.Catholics,can say almost the same thing. Ultimately we are either going to be in heaven or hell,period. Those in purgatory are saved-they're on the way and being purified.Purgatory is NOT a punishment. If anything it is an act of mercy of God.
Non Catholics also have a problem with Catholics praying for the dead. It's related to purgatory. The souls in heaven don't need prayer and the souls in hell are beyond prayer so praying for the dead means we're praying for the souls who may be in purgatory. Only the existence of purgatory makes sense of praying for the dead. Although the Hebrews prayed for the dead and we'll get into that.
It's why the Protestant revolt(if you're Catholic) that is, movement/reformation(if you're Protestant) removed the book of Maccabees from the Bible.
A non Catholic has a valid point in saying purgatory is not mentioned in scripture. The problem is we can say that about the Trinity and Incarnation. A teaching may be there.It doesn't mean the word or term is there.Nor does it mean Jesus didn't teach it either. There are a few scant verses about the Trinity and Jesus never explicitly said,this is the doctrine of the Trinity.
We can say the same about the Incarnation. The verses are there though. The only thing missing is the term Incarnation but there isn't a Christian who doesn't know what the Trinity or Incarnation means. For that matter the word BIBLE isn't in scripture.
Let's delve into Scripture then and find purgatory.
St Paul tell us, "He will suffer loss, though he himself will be saved, but only as through fire" (1 Cor 3:15).
Luke 16:19-31 is interesting because it is a parable taught by Jesus in which he uses an actual name,Lazarus.Lazarus,the rich man,feels compassion and wants his brothers warned. If he were in hell he would feel no compassion.If he were in heaven he would not be suffering.
The Book of Revelation is the clincher because we are told nothing unclean may enter heaven.
Purification would prepare us,if not in this life,in the next.
I have to make this clear. Catholics do not believe in sola scriptura or sola fide. That is,scripture alone or faith alone.
For one we do believe you can lose your salvation. There are too many warnings in scripture to think otherwise. Adam and Eve were created in innocence and through them sin and death entered the world. They were free from sin.NONE of us are totally innocent. They were. If they can fall,surely we're not immune. The angels fell. That alone should be warning enough. We know from our own human experience that we can fall. A non Catholic might say that person was never REALLY saved or never REALLY had faith in the first place.Another words you have to intellectually convince yourself of your own salvation and it becomes the doctrine of the REALLY saved or the REALLY saving faith. A Catholic believes that what God says He will do,He will do. There's more certainty in Catholicism then-you do not have to intellectually determine your own state. I don't judge myself.St Paul said the same thing.
Finally,we do believe in Sola Gracia. Grace alone. As for scripture-we believe it is the INFALLIBLE word of God and has an INFALLIBLE interpreter. Scripture interpretation is not a matter of opinion where everyone is left to their own devices.We know that does not always work out well.It accounts for the various interpretations across denominations .Denominations-outside of Catholicism and its unity-can seldom get beyond Jesus is Lord.