In Calvinism, election to salvation cannot be denied. This is because election is seen as an unconditional act of God's grace, not something that is dependent on the individual's choice. In other words, God has already chosen those who will be saved, and there is nothing that those individuals can do to change that.
This does not mean that the individual's choice is irrelevant. In fact, Calvinists believe that God's grace always works through the individual's free will. However, the individual's choice is not the ultimate factor in salvation. Rather, it is the response to God's grace that is essential.
So, could Paul have just said "thanks, but no thanks"? In Calvinism, the answer is no. Paul was one of the elect, and as such, he was irresistibly drawn to faith in Christ. His choice to believe was not something that he could have simply denied.
This may seem like a difficult concept to understand, but it is one that is central to Calvinism. For Calvinists, salvation is entirely the work of God, and it is something that is ultimately beyond our control. However, this does not mean that we are not responsible for our own choices. Rather, it means that our choices are always made within the context of God's grace.
As Spurgeon said, "I cannot resolve the apparent contradiction that salvation is all of God, but also that sinners must choose to believe." But he also said, "I know both are true because the scripture says salvation is, 'by God's grace,' and 'through the sinner's faith.'"
This is the paradox of Calvinism: salvation is both entirely the work of God and entirely the work of the individual. It is a mystery that we may never fully understand, but it is one that we can embrace with faith.