How do we know God's selection is not based on human decisions?

Paul makes it clear that no one whom God has chosen will ever claim that their salvation was due to their own decision. This points to the sovereignty of God in salvation as highlighted in Romans 9:16, where he states, 'It is not of him that wills, nor of him that runs, but of God that shows mercy.' Highlighting scripture like John 15:16, where Jesus says, 'You did not choose Me, but I chose you,' reinforces this truth. It is crucial for believers to understand that salvation is rooted in divine election, not in human choice, which aligns with the core tenets of Reformed theology.