Christian studies: But it is the children of the promise who are regarded as Abraham's offspring

When Paul refers to the children of promise, he is referring to people converted to the Kingdom of heaven. These people are the receiver of the Beatitudes of the Sermon on the Mount:
"Blessed are the poor in spirit, for theirs is the kingdom of heaven.
Blessed are those who mourn, for they will be comforted.
Blessed are the meek, for they will inherit the earth.
Blessed are those who hunger and thirst for righteousness, for they will be filled.
Blessed are the merciful, for they will be shown mercy.
Blessed are the pure in heart, for they will see God.
Blessed are the peacemakers, for they will be called children of God.
Blessed are those who are persecuted because of the righteousness, for theirs is the kingdom of heaven.
Blessed are you when people insult you, persecute you and falsely say all kinds of evil against you because of me.", Matthew 5:3-11.
Paul is explaining to us that God's blessings are addressed no longer to those who follow the law of Moses, but rather those who follow the freedom of the spirit: "In other words, it is not the children by physical descent who are God's children, but it is the children of the promise who are regarded as Abraham's offspring.", Romans 9:8.

In practical terms, the Apostle Paul is explaining that the gift of eternal life (contemplative activity) and the fruits of the spirit are the blessings of those who follow the path of Jesus, the path of mercy, of piety. Moses' law no longer serves us for justification, those who live according to the law must be judged according to the same.
According to Christian teachings the Kingdom of God is realized in Jesus: "I am the bread of life", John 6:35; "I am the door", John 10:9; "I am the good shepherd", John 10:11; "I am the way, the truth and the life" John 14:6.
Following this logic, the descendants of Abraham are no longer the physical children but the children of the spirit, the children who are born again: "No one can enter the kingdom of God unless they are born of water and the Spirit.", John 3:5.
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