What is the basis of our sonship according to the Bible?

The Bible teaches that our sonship is not founded on human will or effort, but rather on God's sovereign mercy and grace. Romans 9:15-16 illustrates this point, where God declares His right to show mercy according to His will. Paul emphasizes that our belonging to God's family is based solely on what God has done for us in Christ, highlighting the centrality of grace over works. This truth liberates believers from the burden of self-effort, allowing them to rest in the sufficiency of Christ's righteousness and the assurance of their identity as God's children.