They are the saints of old, and the saints that lived through the first century. The Messiah told the High Priest - “You will see me return in the clouds of Heaven - with power and authority.” He said in Revelation - “Those that pierced me would see my return.”
There was great persecution in the 1st century - so much so Paul instructed believers not to worry about marriage or gaining wealth. Not only did the Jews persecute the believers, but also the gentiles. Nero - in particular - whose name adds up to 666 - violently persecuted believers for 3.5 years. Leading to this verse:
Revelation 6:10-11
10 They shouted to the Lord and said, “O Sovereign Lord, holy and true, how long before you judge the people who belong to this world and avenge our blood for what they have done to us?” 11 Then a white robe was given to each of them. And they were told to rest a little longer until the full number of their brothers and sisters. their fellow servants of Jesus who were to be martyred—had joined them.
The Messiah returned to fulfill the feast of Trumpets through the judgment of Jerusalem - just as he promised he would (He told the Pharisees that the sins of their ancestors will fall on this generation) - then he judged the gentile nations. He (not Constantine) ended the persecution of the saints, and gave them great power and authority in their resurrection to rule over the nations:
Revelation 2:26-27
To the one who is victorious and does my will to the end, I will give authority over the nations— 27 that one ‘will rule them with an iron scepter and will dash them to pieces like pottery just as I have received authority from my Father.
The period known today as the dark ages - was the period when the saints were ruling on the earth - with their glorified bodies. After that period ended - the saints in glorified bodies left - and Satan was released from his prison to deceive the nations. Satan’s job is to set the table for us to choose between life and death.