Purgatory again. Even though it's an ancient pagan practice adopted by your church, you keep defending it. In what sense is purgatory a "practice"?
Anyway, if what you're saying is that there were similar beliefs among pagans, I don't see how that automatically makes it wrong. The idea of a god who dies and comes back to life existed among various pagan religions, so would you say that this means that the Resurrection is a pagan idea adopted by the Church?
Anyway, in 1 Corinthians 3, you take verse 13 ("Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is.") and claim that the work being tried by fire is everything that we could be judged about, and that therefore a purging in purgatory is necessary to burn away our unforgiven sins I never said anything about unforgiven sins; I'm talking about the temporal punishment for sins. Even after a sin is forgiven, a punishment can remain. For instance, in 2 Samuel 12, David is punished even though his sin was already forgiven:
2 Samuel 12:13-14
And David said unto Nathan, "I have sinned against the Lord." And Nathan said unto David, "The Lord also hath put away thy sin; thou shalt not die. Howbeit, because by this deed thou hast given great occasion to the enemies of the Lord to blaspheme, the child also that is born unto thee shall surely die."
It is blatantly obvious that the works being discussed throughout chapter 3 of 1 Corinthians are in the context of the Corinthians being divisive about whether they followed Paul, or Apollos, or another. Sure. But Paul goes on to talk about what happens on Judgment Day, based on what we do in this life, and this is applicable to anyone even though he's speaking about particular people here. "Every man shall receive his own reward according to his own labour. For we are labourers together with God."
Then Paul notes that he was tasked by God with being the masterbuilder who constructs the foundation for churches. Then he warns to build upon that foundation with care; that only building upon Christ is right, not building on one teacher or another. Well, he says that there is only one foundation that can be laid: Christ. "For other foundation can no man lay than that is laid, which is Jesus Christ." It's not teachers who are being built on, it's teachers who are doing the building. And it is based on how well they build that they will be judged: "But let every man take heed how he buildeth thereupon."
Building with gems, gold, etc. is not proper. Building with gold, silver, precious stones is proper. What is improper is building with wood, hay, and stubble. The first group of building materials are high-quality materials, and they will survive the fire; but the second group won't survive. "Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is. If any man's work abide which he hath built thereupon, he shall receive a reward. If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire." This last part describes purgatory, here with respect to the teachers, but it also applies to any Christian, since we will all be judged, not just the teachers. Once we are saved, we are temples of God (1 Corinthians 6:19). Our foundation, as Christians, is Christ. We work with God in building up the temple. If we do a good job, we go to heaven. If we do a bad job, our work will be purified of whatever was bad, but we will ultimately go to heaven. On the other hand, if we defile the temple, losing the grace of justification, then we will go to hell: "If any man defile the temple of God, him shall God destroy."
And one more point; of course Jesus is and was sinless because He is God; that is the divine portion of His nature. Yet He is not only 100% God, He is also 100% man. And as a man, if He was born in the typical fashion, He would have been tainted in His human portion by being a descendant of Adam. Only the virgin birth cleared Him of Adam's seed. That is why in Genesis, His birth is fortold as "the seed of the woman". Women don't have the seed, that's the male contribution. So only Christ was sinless, as only He was from the seed of a woman. What you say here isn't true. The Bible describes other women as having "seed". For instance:
Genesis 16:10
And the angel of the Lord said unto her, "I will multiply thy seed exceedingly, that it shall not be numbered for multitude."
Genesis 24:60
And they blessed Rebekah, and said unto her, "Thou art our sister, be thou the mother of thousands of millions, and let thy seed possess the gate of those which hate them."