According to the teaching of Unconditional Election, God has already chosen who is saved and who is condemned. If we have no control over whether we receive or reject grace, someone can never truly know if they are saved or condemned. Therefore why does a Calvinist continue to worship and serve God if they don't know if they will be saved? Aren't they worshiping God in vain?
If we have no control over whether we receive or reject grace, someone can never truly know if they are saved or condemned.
Reformed theology teaches the doctrine of assurance of salvation, in addition to the doctrine of unconditional election. This doctrine teaches that we CAN know if we are truly saved or condemned.
The basics of this doctrine can be found in the Canons of Dort:
Canons of Dort, I.12 and V.220.127.116.11
The elect in due time, though in various degrees and in different measures, attain the assurance of this their eternal and unchangeable election, not by inquisitively prying into the secret and deep things of God, but by observing in themselves with a spiritual joy and holy pleasure the infallible fruits of election pointed out in the Word of God such as, a true faith in Christ, filial fear, a godly sorrow for sin, a hungering and thirsting after righteousness, etc.
It can also be found in the Heidelberg Catechism:
Q21: What is true faith?
A21: True faith is not only a sure knowledge, whereby I hold for truth all that God has revealed to us in His Word, but also a hearty trust, which the Holy Ghost works in me by the Gospel, that not only to others, but to me also, forgiveness of sins, everlasting righteousness, and salvation are freely given by God, merely of grace, only for the sake of Christ's merits.
It can also be found in the Westminster Confession of Faith, in Chapter XVIII: "Of Assurance of Grace and Salvation".
Essentially, by seeing in ourselves the signs of salvation, we can know that we are saved. Based on this knowledge, we can from there continue to follow the commandments of God in the assurance that our worship is not in vain.
It could be also argued that it is in every way preferable to continue to worship and obey God even when we doubt our own salvation.
If you are elected, you will not want to sin or dwell in gross darkness. One is not saved because their works are good; their works are good because they are saved.
Acts 13:48 And when the Gentiles heard this, they were glad, and glorified the word of the Lord: and as many as were ordained to eternal life believed.
Irresistible grace is the logical following, insomuch as their will is conducive towards God's good pleasure. They cannot lose their salvation and they cannot resist it, for this is the eternal security of the believer.
Philippians 2:13 For it is God which worketh in you both to will and to do of his good pleasure.
Matthew 7:18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
Nobody who is truly saved will be permitted to dwell in darkness indefinitely. A good tree cannot bring forth evil fruit. It's an axiomatic matter of fact. The debt is paid and the sins remitted. The worshipping of God is not in vain; Christ's blood was effacacious for those ordained to eternal life, and the will to worship God is necessitated by one's salvation, rather than rendered obsolete or excused.
Job 1 says this:
8 Then the Lord said to Satan, “Have you considered my servant Job? There is no one on earth like him; he is blameless and upright, a man who fears God and shuns evil.”
9 “Does Job fear God for nothing?” Satan replied. 10 “Have you not put a hedge around him and his household and everything he has? You have blessed the work of his hands, so that his flocks and herds are spread throughout the land. 11 But now stretch out your hand and strike everything he has, and he will surely curse you to your face.”
Why do I cite this? Your question is very specific: why should someone worship God if they do not thereby receive any assurance of salvation? My answer will encompass this and every other question of the kind, "Why worship God if doing so will not earn me the blessing of - fill in the blank - ?" Assurance of salvation is just one such blessing. Others include long life, health, wealth, wisdom, respect, a good name, a happy family, etc. Job was systematically denied every benefit except life, yet he persevered in his faith. He continued to proclaim God's character, justice, mercy and greatness to his friends, even while they attacked him for being a sinner deserving punishment.
In Job 19,
“Oh, that my words were recorded, that they were written on a scroll, 24 that they were inscribed with an iron tool on lead, or engraved in rock forever! 25 I know that my redeemer lives, and that in the end he will stand on the earth. 26 And after my skin has been destroyed, yet in my flesh I will see God; 27 I myself will see him with my own eyes—I, and not another. How my heart yearns within me!
Job held fast to an assurance of God's justice and mercy so powerful that even the dire circumstances of his life could not rob him of it. He did not have a written Bible or a legal contract or a prophecy or miracle to hold onto. His faith and continued worship of God in the absence of all those supports WAS HIS ASSURANCE. The kind of God that can reach through to us in the midst of the worst this world can offer is WORTHY of being worshiped. The kind of person that can love and worship God from such a place of pain offers a gift worthy of being called worship.