How do we know Christ's death was for a specific group of people?

Throughout the New Testament, passages such as Romans 8:28-30 and John 6:37 clarify that Christ's death is aimed at those who have been called by God. The notion of effectual atonement is rooted in the belief that if Christ died for someone, then that individual's sins are paid for, and they will certainly be brought to faith. This assurance comes from the nature of God's promise, where His love and election are demonstrated through the effectiveness of Christ's work on the cross, ensuring that no charge can be brought against those for whom He died. Furthermore, the use of the term "us" in Romans reflects the specificity of His redemptive work.