The parable is told in verses 28-30
The summation of the parable is told in verses 31-32.
The summation Jesus gave, after He told the parable, was that the tax collectors and prostitutes who believed, would enter the Kingdom ahead of the Pharisees.
28 “What do you think? A man had two sons; he went to the first and said, ‘Son, go and work in the vineyard today.’
29 He answered, ‘I will not’; but later he changed his mind and went [tax collectors and prostitutes].
30 The father went to the second and said the same; and he answered, ‘I go, sir’; but he did not go.[Pharisees]
End of parable.
Summation of parable:
31 Which of the two did the will of his father?” They said, “The first.” Jesus said to them, “Truly I tell you, the tax collectors and the prostitutes are going into the kingdom of God ahead of you.
32 For John came to you in the way of righteousness and you did not believe him, but the tax collectors and the prostitutes believed him; and even after you saw it, you did not change your minds and believe him.
Jesus said the tax collectors and the prostitutes (believers) are going into the kingdom of God ahead of them. Not instead of them. But ahead of them.
Actually MMX the scripture contexts you have left out here absolutely says that the Tax collectors and the prostitutes in reference to the Scribes and the Pharisees going in ahead of them is in reference to the Scribes and the Pharisees not entering into the Kingdom of Heaven. Here let's look at all the context of the scriptures you quoted.
Matthew 21:28-45
v28-30. parable of the man with two sons the one who said no to the father but went out and did the fathers will and the other who said yes but did not do what the father asked.
v31. The one that said no but did what the father asked representing the tax collectors and the prostitutes and the other saying yes representing the Scribes and the Pharisees and the tax collectors and the prostitutes going into the kingdom of God ahead of the Scribes and the Pharisees.
v32. John the baptist came in the way of righteousness
but the Scribes and Pharisees did not believe him.
v33. Parable of the vineyard lent out to the farmers (Israel born of the flesh) of the householder
v34. At the seasons end the householder sent his servants to receive the fruit of the harvest .
v35. The farmers took the householders servants (the prophets) and beat them and killed them
v36. The householder sent more servants that the farmers did the same to them as the first
v37. The householder then sent his son thinking they will respect my son
v38. The farmers saw the son and said quick lets kill him and take his inheritance
v39. They caught the son and killed him.
v40. The question is asked what will the householder do to those farmers at his return?
v41. The Scribes and Pharisees answered him he will miserably destroy those wicked men and give his vineyard to others.
v42. Jesus answered, Did you never read in the scriptures, The stone which the builders rejected, the same is become the head of the corner: this is the Lord's doing, and it is marvelous in our eyes?
v43-45. Therefore say I to you,
The kingdom of God shall be taken from you, and given to a nation bringing forth the fruits thereof. And whoever shall fall on this stone shall be broken:
but on whomsoever it shall fall, it will grind him to powder. And when the chief priests and Pharisees had heard his parables, they perceived that he spoke of them.
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The reason why the Tax collectors and the prostitutes went into the Kingdom of God ahead of the Scribes and the Pharisees is that they believed and followed Gods' Word where as the Scribes and the Pharisees did not believe and follow Gods' Word. Because the Scribes and the Pharisees did not believe and follow Gods' Word Jesus says that the kingdom of God was to be taken from them and given to another. Those who believe and follow Gods' Word. Therefore when Jesus said earlier that the Tax collectors and the prostitutes go in to God's kingdom "ahead of you" he was saying if you do not believe and follow Gods' word the kingdom of God will be taken away from you. Your not going there.
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All the above also agreeing with what Jesus says here...
John 3:36 He that believes on the Son has everlasting life: and he that believes not the Son
shall not see life; but the wrath of God stays on him.
It is very clear from the scriptures here that is we do not believe and follow God's Word we will not receive eternal life. Gods' promises therefore are conditional therefore on believing and following what God's Word says.
Hope this is helpful.