How do we know that God's plan for salvation includes all people?

Ephesians 3:9-10 reveals that God's plan is for 'all men' to understand the fellowship of the mystery hidden in Christ, illustrating that from the beginning, the plan of redemption was unified and included all humanity. The idea that God would momentarily set aside His original plan for salvation is contrary to His immutable nature; He does not change His mind (Malachi 3:6). This truth assures us that the gospel is intended for a diverse group of believers, as Paul emphasizes in his letters that both Jews and Gentiles are included in the covenant of grace.