I've often used what I call the “nutshell” argument for purgatory: we must be without sin to enter into God's presence (Eph 5:5; Heb 12:14; Rev 21:27; 22:3, 14-15). Therefore, God must purge or wash away our sin to make us fit to be in heaven with Him. All agree so far. The only disagreement is whether this “divine cleansing” takes place in an instant or is more of a process. It's merely a quantitative difference; not an essential one. Purgatory is indicated most directly in 1 Corinthians 3:13, 15 (RSV):

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. . . . [15] If any man's work is burned up, he will suffer loss, though he himself will be saved, but only as through fire.

The Bible also often refers to this same purging process taking place before we die: the very common biblical theme of God's chastising or purifying His people. By analogy, this shows us the same notions that lie behind the apostolic and Catholic doctrine of purgatory (methods of how God works, so to speak). When these passages are included, one can find (as I did) as many as fifty biblical passages that are relevant to purgatory.

Scripture refers to a purging fire (in addition to 1 Corinthians 3 above): whatever “shall pass through the fire” will be made “clean” (Num 31:23); “Out of heaven he let you hear his voice, that he might discipline you; and on earth he let you see his great fire, and you heard his words out of the midst of the fire” (Dt 4:36); “we went through fire” (Ps 66:12); “For gold is tested in the fire, and acceptable men in the furnace of humiliation” (Sir 2:5); “our God is a consuming fire” (Heb 12:29); “do not be surprised at the fiery ordeal which comes upon you to prove you” (1 Pet 4:12); We also see passages about the “baptism of fire” (Mt 3:11; Mk 10:38-39; Lk 3:16; 12:50).

The Bible makes frequent use also of the metaphor of various metals being refined (in a fire): “when he has tried me, I shall come forth as gold” (Job 23:10); “thou, O God, hast tested us; thou hast tried us as silver is tried” (Ps 66:10); “The crucible is for silver, and the furnace is for gold, and the LORD tries hearts” (Prov 17:3); “I will turn my hand against you and will smelt away your dross as with lye and remove all your alloy” (Is 1:25); “I have refined you, . . . I have tried you in the furnace of affliction” (Is 48:10); “I will refine them and test them” (Jer 9:7); “I will put this third into the fire, and refine them as one refines silver, and test them as gold is tested” (Zech 13:9);  “he is like a refiner's fire, and like fullers' soap; he will sit as a refiner and purifier of silver, and he will purify the sons of Levi, and refine them like gold and silver” (Mal 3:2-3); “Having been disciplined a little, they will receive great good, because God tested them and found them worthy of himself; [6] like gold in the furnace he tried them, . . . “ (Wis 3:5-6); “. . . your faith, more precious than gold which though perishable is tested by fire” (1 Pet 1:6-7). God cleansing or washing us is another common biblical theme: “Wash me thoroughly from my iniquity, and cleanse me from my sin! . . . Purge me with hyssop, and I shall be clean” (Ps 51:2, 7); “Blows that wound cleanse away evil; strokes make clean the innermost parts” (Prov 20:30; cf. 30:12); “the Lord shall have washed away the filth of the daughters of Zion and cleansed the bloodstains of Jerusalem from its midst by a spirit of judgment and by a spirit of burning” (Is 4:4);  “I will cleanse them from all the guilt of their sin against me” (Jer 33:8); “I will sprinkle clean water upon you, and you shall be clean from all your uncleannesses” (Ezek 36:25); “cleanse them from sin and uncleanness” (Zech 13:1);  “our hearts sprinkled clean from an evil conscience and our bodies washed with pure water” (Heb 10:22); “he was cleansed from his old sins” (2 Pet 1:9); “the blood of Jesus his Son cleanses us from all sin” (1 John 1:7).

Divine “chastisement” is taught clearly in many passages:  “as a man disciplines his son, the LORD your God disciplines you” (Dt 8:5); “do not despise the LORD's discipline or be weary of his reproof,” (Prov 3:11); “I will chasten you in just measure” (Jer 30:11); “For thou didst test them as a father does in warning” (Wis 11:10); “God who tests our hearts” (1 Thess 2:4); “For the Lord disciplines him whom he loves, and chastises every son whom he receives. It is for discipline that you have to endure. God is treating you as sons; for what son is there whom his father does not discipline? . . . he disciplines us for our good, that we may share his holiness” (Heb 12:6-7, 10).

We are subject to God's indignation or wrath, insofar as we sin: “God will bring every deed into judgment” (Ecc 12:14); “I will bear the indignation of the Lord because I have sinned against him, . . . He will bring me forth to the light” (Mic 7:9).

Purgatory is “written all over” the passages above. I once didn't make the connection of what seems so obvious to me now. I think there are many who (like myself)  may be able to be persuaded to see that the Bible is far more “Catholic” than they had ever imagined.