According to Calvinism, God's choice is irrevocable. If you are chosen, no one can say you are not. No matter what you do or who you seem to be, God's choice is solid, as real as a mountain, as just as immovable. Even more.
And this is true even if you yourself don't choose it. God chooses you, you do not necessarily choose Him. God is the actor, we are the recipient. And if we try to refuse God, it doesn't matter. God's choice is from the beginning of the world, and He will hunt you down like the Hound of Heaven He is. This is known as irresistible grace. God is going to bless you, if you are chosen, like it or not. After all, He's bigger than we are.
It is true, Scripturally, that it is God who is the actor and that He is the initiator/seducer in our romance with Him. Jesus said, "All that the Father gives Me will come to Me, and the one who comes to Me I will certainly not cast out." and more specifically, "No one can come to Me unless the Father who sent Me draws him.". (John 6:44, 37)
The proof texts for God's choice being irresistible are these:
"My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand. My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father's hand." (John 10:27-29)
"For those whom He foreknew, He also predestined to become conformed to the image of His Son, so that He would be the firstborn among many brethren; and these whom He predestined, He also called; and these whom He called, He also justified; and these whom He justified, He also glorified. What then shall we say to these things? If God is for us, who is against us? He who did not spare His own Son, but delivered Him over for us all, how will He not also with Him freely give us all things? Who will bring a charge against God's elect? God is the one who justifies; who is the one who condemns? Christ Jesus is He who died, yes, rather who was raised, who is at the right hand of God, who also intercedes for us. Who will separate us from the love of Christ? Will tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword? Just as it is written, "FOR YOUR SAKE WE ARE BEING PUT TO DEATH ALL DAY LONG; WE WERE CONSIDERED AS SHEEP TO BE SLAUGHTERED." But in all these things we overwhelmingly conquer through Him who loved us. For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord." (Rom 8:29-39)
The Romans 8 passage has it all: God's choice, God's grace, and the fact that no one can take that choice away.
However, in both passages, the point is not that no one can resist what God chooses, like it or not, but that no one can claim that God's choice is null and void. The context in Romans 8 is one of persecution. Religious zealots may claim that some-- namely Gentiles-- are not chosen of God, and that they deserve to be punished and shamed because of their claim to be of God. However, Paul is saying that no one can take God's choice away. The choice is indicated by the Spirit of God (as said earlier in the chapter) and no one can claim otherwise, and punishment or shame will not take that away.
But can one chose to reject God's call? That is certainly shown in Scripture:
"I know your deeds, that you are neither cold nor hot; I wish that you were cold or hot. So because you are lukewarm, and neither hot nor cold, I will spit you out of My mouth. Because you say, "I am rich, and have become wealthy, and have need of nothing," and you do not know that you are wretched and miserable and poor and blind and naked, I advise you to buy from Me gold refined by fire so that you may become rich, and white garments so that you may clothe yourself, and that the shame of your nakedness will not be revealed; and eye salve to anoint your eyes so that you may see. Those whom I love, I reprove and discipline; therefore be zealous and repent. Behold, I stand at the door and knock; if anyone hears My voice and opens the door, I will come in to him and will dine with him, and he with Me. He who overcomes, I will grant to him to sit down with Me on My throne, as I also overcame and sat down with My Father on His throne." (Revelation 3:15-21)
This passage is the letter to the church of Laodicea. They are of the church, and so believers, but Jesus claims that they are on the verge of being "spit out" because of their shame. Jesus begs them to open the door to him, and those who "overcome" their own sin will be in the kingdom of God. But those who do not chose this way will be rejected. It's all about choice.
In Hebrews there's a more interesting passage:
In the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame. (Hebrews 6:4-6)
Not only can one who, by all accounts, has been saved fall away, but they cannot come back!
In general, given the focus of repentance as being a presupposition to living a life in the kingdom, I'd say that God's choice is essential, but so is our own. God's choice and our choice works together to bring God's people together into one nation.
