… having also believed, you were sealed in Him with the Holy Spirit of promise” (Eph 1:13)
The Bible tells you how God’s angel is sealing righteous in the last days. It is clear from the Ephesians that the seal can be obtained much earlier in each person’s life. Eph 1:13 In him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in him with the Holy Spirit of promise.
The seal of the Holy Spirit is given to a person once he has heard the truth, the gospel, and become a believer. Hearing the truth also means calling of God and becoming a believer means to be chosen.
Eph 4:30 Do not grieve the Holy Spirit, by whom you were marked with a seal for the day of redemption.
How does obtaining the seal be related to the so-called great multitude? Rev 7:9 After these things I looked, and there was a large crowd that no one was able to count! They were from every nation, tribe, people, and language. They were standing in front of the throne and the lamb and were wearing white robes, with palm branches in their hands. 7:14 I said to him, My lord, you know. And he said to me: these are those coming out of the great tribulation; and they washed their robes and whitened them in the blood of the Lamb.
In the great tribulation and in the Lord’s day of the last days most of the mankind will be destroyed. However, there will be a great many survivors. They may have come to repentance and faith in the very end. According to Jesus’ parable, He pays everyone the same pay regardless of whether the employee has been working all day or just a short while before the end of the day. Similarly, man can be saved at the very last moment by calling the Lord’s name. In my own interpretation, this way, they will be saved, but they save only their lives, and only after they have proved to be true believers and righteous, they receive eternal life in the final judgment. Salvation can happen in the same way as the Ephesian letter tells of receiving the seal of God.