Re: called vs. chosen:
God called (summoned) everyone to be holy. But He also chose out a people for Himself, and He dedicated them to Himself, just like He chose the priests and Levites of Israel to do special service to Himself. Not all of the people He called out of Egypt actually went; and not all of the people He called to Mount Sinai actually stayed His people.
But He still called all the Gentiles to find out about Him, and eventually He gave them enough knowledge for many of them to become Christians. Those Gentiles who obeyed Him were chosen to be His servants, just as He had said at Isaiah 66:21 –
"And some of them also I will take for priests and for Levites," says the LORD.
That is the original distinction between called and chosen.
Now, in the passage of Matthew that you are quoting, Jesus gives this saying His own twist. He says that the King invites large numbers of people to His Son’s wedding feast, but that most of His guests refuse to come and even kill his servants. He dispatches his armies to “kill those murderers and destroy their cities.” So he invites random people that his servants find on the roads.
But one of the beggars who comes refuses to get wedding garments before coming. (It was common for poor people or relatives to borrow wedding garments from those more prosperous, and often rural synagogues had a sort of common store of wedding garments for everybody to use. So a guest not wearing a wedding garment was a rude lack of preparedness, not a sign of poverty.) So the King throws out the rude guy, and the moral of the story is that “Many are called, but few are chosen.”
Jesus’ twist is that any of the Chosen who don’t answer the call will find out that they are not Chosen at all (or that being Chosen doesn’t benefit them). Responding to the call in the right way, the faithful way, is what makes one really Chosen.
The same thing is true for Christians. Becoming one of the Chosen through baptism will do us no good, unless we answer the King’s invitation in a positive way, a faithful way.
The other usage of this moral of the story is the parable of the vineyard workers, where those who come late get the same wage as those who come early.
Matthew 20:13-16 –
Jesus’ twist is that some of the Chosen People will find out that they are no more and no less Chosen than the Gentiles who answered late. And if they do, they have no right to complain and no reason to. God can Choose whomever He feels like, whenever He feels like. And He feels like calling everyone. When the time is right, He makes that call louder.