I haven’t heard Matthew 21:28-32 used to defend Purgatory before, but it seems to me that it does so pretty plainly. Jesus is speaking here to “the chief priests and elders of the people”:
28″But what do you think? A man had two sons, and he came to the first and said, ‘Son, go work today in the vineyard.’
29″And he answered, ‘I will not’; but afterward he regretted it and went.
30″The man came to the second and said the same thing; and he answered, ‘I will, sir’; but he did not go.
31″Which of the two did the will of his father?” They said, “The first.” Jesus said to them, “Truly I say to you that the tax collectors and prostitutes will get into the kingdom of God before you.
32″For John came to you in the way of righteousness and you did not believe him; but the tax collectors and prostitutes did believe him; and you, seeing this, did not even feel remorse afterward so as to believe him.
There are plenty of directions in which to go from reading this. First, the Kingdom of God here refers to Heaven. The tax collectors and prostitutes already believe, but will get into the Kingdom of God, so it’s related to a future state. Second, Christ is explicitly tying salvation to obedient works: acts of faith, rather than simply belief. Third, this supports the notion of so-called “Anonymous Christians,” something usually denied by Protestants, and occassionally by Catholics as well. This is the notion that there are some who are not publicly Christians (and perhaps do not even consider themselves Christians), who still are Christians. Matthew 21:28-32 pretty clearly establishes that you can’t establish who’s damned simply by looking at the external markers.
But the most fundamental thing about the passage is that Jesus is saying that some people, due to their faith, will get to Heaven before others. If Purgatory weren’t real, I have no idea how one would read Matthew 21:31. Jesus obviously doesn’t mean that the tax collectors and prostitutes are going to die sooner. And so, if they’re not going to die sooner, but they are going to arrive in Heaven sooner, doesn’t that suggest that the chief priests and elders are going to be delayed a bit after death? Note also that here, Jesus isn’t suggesting that they’re not going to Heaven, just that they’re going to get there after the tax collectors and prostitutes. In fact, if they’re not getting there at all, I don’t see how v. 31 makes sense.
The term used here, proagō, means to go ahead of someone, and implies that that someone follows you there. So in Matthew 26:32, proagō is the term used when Jesus says, ” I will go before you into Galilee” (same with Mark 16:7, when the prophesy is fulfilled). It implies, of course, that the Disciples are going to Galilee after Him. Even the Star of Bethlehem “went before” the Magi to Bethlehem (Matthew 2:9). So the people Jesus is speaking to are apparently going to Heaven ultimately, just not right away.
Of course, Matthew 21 doesn’t really spell out Purgatory at all. Jesus doesn’t say, “the reason that the tax collectors and prostitutes are going before you is that you need to be painstakingly purified of your sins before you can enter the glories of Heaven.” But nonetheless, the passage seems to make sense only if Purgatory is true.