God created the Sabbath for mankind (Mark 2:27), and there will yet come a time when all people will keep God's Sabbath.
The Bible speaks of God's Kingdom as being established on earth when Jesus Christ returns to rule as "King of Kings and Lord of Lords" (Revelation 11:15; Revelation 19:11-16). In addition to ruling over sons of God who will be changed to spirit beings and rule with Christ in the Kingdom of God at the time of His second coming, Jesus will rule as Head of a literal government over physical nations on earth (Psalm 22:27-28; Psalm 72:1-11; Daniel 2:34-35; Zechariah 14:8-9).
At that time, all nations will be ruled by God's laws (Jeremiah 31:31-34; Micah 4:2; Isaiah 2:2-3; Hebrews 8:7-13).
All of humanity will then learn and keep God's Sabbath: "'From one New Moon to another and from one Sabbath to another, all mankind will come and bow down before me,' says the Lord" (Isaiah 66:23, New International Version).
The Sabbath will be kept by gentiles and those who were never a part of the physical nation of Israel. "Thus says the Lord: 'Keep justice, and do righteousness, for My salvation is about to come, and My righteousness to be revealed. Blessed is the man who does this, and the son of man who lays hold on it; who keeps from defiling the Sabbath, and keeps his hand from doing any evil . . .
"To the eunuchs who keep My Sabbaths, and choose what pleases Me, and hold fast My covenant, even to them I will give in My house and within My walls a place and a name better than that of sons and daughters; I will give them an everlasting name that shall not be cut off.
"'Also the sons of the foreigner who join themselves to the Lord, to serve Him, and to love the name of the Lord, to be His servants—everyone who keeps from defiling the Sabbath, and holds fast My covenant—even them I will bring to My holy mountain, and make them joyful in My house of prayer'" (Isaiah 56:1-2, Isaiah 56:4-7).
Sabbath observance is specifically mentioned as a part of the covenant God will make with all peoples then, and their worship of Him. It will be an important part of Jesus Christ's coming reign on earth!