What are the two acts of God at salvation regarding becoming a son and heir?
At salvation, the Holy Spirit causes the believer to be born into God's family, making them a child of God. Then God performs another act, placing that believer as a full heir and joint heir with Jesus Christ, equal in inheritance.
Les explains that at the moment of salvation, yes, we are born into the family of God by the work of the Holy Spirit. But immediately there comes another act of God: He also places us as the full heir with Jesus Christ. This is not something we strive or strain to attain over many years; it happens the moment we believe. That is why we can say Abba Father. Later, verse 17 says if we are children, then we are heirs, and not just an heir down the line hoping for a little inheritance, but a joint heir. Even in our own culture, a joint heir implies equality, so what is His is hers, and vice versa.