Hebrews 12:28

Hebrews 12:28 Therefore receiving an unshakable kingdom, let us have grace, through which we may serve God well-pleasingly with piety and fear;

the kingdom in its reality, or the reality of the kingdom, is an exercise and a discpline to us in the church today whereas the kingdom in its manifestation, or the manifestation of the kingdom, will be a reward and an enjoyment to us in the millennial kingdom in the coming age. If we take the Spirit's exercise and God's discpline in the reality of the kingdom today, we will receive the Lord's reward and enter into the enjoyment of the coming Sabbath rest in the manifestation of the kingdom in the coming age; otherwise, we will miss the coming kingdom, we will not be rewarded with the manifestation of the kingdom at the Lord's coming back, we will have no right to enter into the glory of the kingdom to participate in Christ's reign in the millennial kingdom, and we will be unable to inherit the earth on the coming age, to be royal priests serving God and Christ as His manifested glory, and to be Christ's co-kings ruling all the nations with His divine authority. To miss the coming kingdom and to forfeit our birthright does not mean that we will perish. it means that we will lose the reward but not our salvation.