The gospel of the kingdom of God is good news for all of mankind, regardless of when or where people lived. The good news, or gospel, is that Jesus came to save everyone.
John 3:17 New King James Version
17 For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.
John 12:46-47 Good News Translation
46 I have come into the world as light, so that everyone who believes in me should not remain in the darkness. 47 If people hear my message and do not obey it, I will not judge them. I came, not to judge the world, but to save it.
Those who lived before Jesus died for them, will have their first opportunity to learn about Jesus and decide if they want to follow him when they are resurrected back into a mortal life. Those who lived after Jesus shed his blood for the remission of sin, but did not have the opportunity to learn about Jesus, will also have their opportunity to accept him when that same resurrection takes place, during their second physical life. God said this resurrection would take place in the future and that those who were resurrected would finally be able to live in a world ruled with justice and love. What do you know about that resurrection? When only believing in Jesus can bring eternal life, how can those who didn’t get to believe in Jesus, receive eternal life? When do they get to hear and believe? For them to be saved, God must provide some way for them to hear and believe in Jesus. How does God provide that way? Provide scriptural proof.
Habakkuk 2:14 Amplified Bible
14 “But [the time is coming when] the earth shall be filled With the knowledge of the glory of the Lord, As the waters cover the sea.
Isaiah 11:4-9 New International Reader’s Version
4 He will always do what is right when he judges those who are in need. He’ll be completely fair when he makes decisions about poor people. When he commands that people be punished, it will happen. When he orders that evil people be put to death, it will take place. 5 He will put on godliness as if it were his belt. He’ll wear faithfulness around his waist. 6 Wolves will live with lambs Leopards will lie down with goats. Calves and lions will eat together. And little children will lead them around. 7 Cows will eat with bears. Their little ones will lie down together. And lions will eat straw like oxen. 8 A baby will play near a hole where cobras live. A young child will put its hand into a nest where poisonous snakes live. 9 None of those animals will harm or destroy anything or anyone on my holy mountain of Zion. The oceans are full of water. In the same way, the earth will be filled with the knowledge of the Lord.