Jesus said this about himself. The thing is, this means that meaningful [actual] resurrection is to be encountered in THIS life, not so much the next. He appeared 2000 years ago and made this statement, and then followed it up with his own resurrection.
So we might say the resurrection was over, as some apparently wrongly said then. Christians are those who die with him in baptism, [death to the old life] and are raised again through faith in Him. They become participants in the resurrection, being HIS resurrection, which now by faith includes THEM. They become new creations NOW, even while still in this mortal body. They are “resurrected”. [from the dead, from amongst the “dead”.]
They are said to be raised with Him in the heavenlies, where He is seated at the right hand of God. Such is the authority now with Christians. So as each individual encounters the gospel in this present life, they choose or not to become at one with Jesus in His resurrection, to be “in” Him, or not. (Or to be without Him, [outside him] not “in” Him). [And Him not in you, “Christ in you, the hope of glory”].
The writing in the book of Revelation then becomes not a matter so much of futuristic resurrection, but one of present day judgement which hinges on our present day acceptance or rejection of Him. “THIS is the judgement…” (John). The words which mention the “First Resurrection” are talking about Jesus and his followers, not necessarily some chronological series of events. The last day resurrection simply confirms what state the person was in at physical death, whether or not they were “spiritually” (actually) alive. If not in Christ, they suffer the “second death”.
“Those who take part in the first resurrection” are those who have been “born again” to the living hope that Jesus is. “They will not be harmed by the second death”. Only those who take part in the resurrection of life in Jesus, will be the ones who take part in the resurrection of life at the end. The others are resurrected to death, the death they have clung to instead of surrendering it to Him.
Being born again in Christ, becoming a “new creation” in Him, IS the “First Resurrection” for us, of us. To be confirmed when He returns.