United in Worship book pg 114:
Spiritual
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God’s spirit gives positive assurance of adoption as spiritual sons to baptized Christians who have received the heavenly calling. The apostle Paul showed this when he wrote to the “holy ones” in Rome, describing what was at that time the situation of all true Christians. He said: “All who are led by God’s spirit, these are God’s sons. For you did not receive a spirit of slavery causing fear again, but you received a spirit of adoption as sons, by which spirit we cry out: ‘Abba, Father!’ The spirit itself bears witness with our spirit that we are God’s children. If, then, we are children, we are also heirs: heirs indeed of God, but joint heirs with Christ, provided we suffer together that we may also be glorified together.”—Rom. 1:7; 8:14-17.9
Two uses of the word “spirit” are here brought to our attention: “the spirit itself” and “our spirit.” The first is God’s invisible active force. It inspires in his spiritual children a conviction of having been adopted as God’s free children. That spirit also bears witness through God’s inspired Word, the Bible, which is like a personal letter to his spiritual children. (1 Pet. 1:10-12) When those who have been begotten by holy spirit read what the Scriptures say to those who are spiritual sons of God, they properly respond: ‘That applies to me.’ Thus God’s own active force in various ways bears witness with their spirit, the motivating force of their own mind and heart, that they are God’s children. In accord with what God’s spirit thus indicates, their minds and hearts become set on the prospect of being joint heirs with Christ, and they accept the responsibilities of God’s spiritual children.—Phil. 3:13, 14.