Which concept describes the achievement of spiritual sonship in a believer?

The concept that describes the achievement of spiritual sonship in a believer is adoption. In Christian theology, adoption refers to the process by which individuals are accepted into God's family as His children, established through faith in Jesus Christ. This means that believers are granted the legal status and privileges of being part of God's family, akin to how a child legally becomes part of a family through adoption.

Adoption signifies a transformation of identity; believers are not just forgiven of their sins or justified before God, but they are also welcomed into an intimate relationship with God as their Father. This relationship indicates a deep assurance of belonging and acceptance that comes from being chosen and loved by God.

Regeneration, while vital, refers to the process of being spiritually reborn or given a new life in Christ, focusing more on the internal change that prepares a believer to enter into a relationship with God. Justification deals primarily with God's declaration of righteousness over the believer, cleaning them of sin and enabling a right standing before God, but it does not specifically address the relational aspect of being a child of God. Redemption involves the act of being freed from sin and its consequences, emphasizing the rescue aspect of salvation but not directly encompassing the relational dimension of being adopted into God's family.