Revelation 7

So he said to me, “These are the ones who come out of the great tribulation, and washed their robes and made them white in the blood of the Lamb. v.14

Read chapter 7
In this chapter we see a break from the destruction and horror going on in the tribulation and read of all those that are sealed receiving, not the mark of the beast as we'll read about later in Revelation, but the mark of God on their foreheads (v.3).  This is the chapter where you may have heard of the 144,000 are saved, but note that that's not it of everyone whose ever lived, that's just of Israel for a multitude which no one could number will be saved as well as we read in verse nine of this same chapter.  And those that are given the white robes are the ones who are raised to life again.  This is a symbol of their lives purified by dependance of sacrifice of Jesus.
If you are a believer in Jesus Christ as your savior today, you are already sealed by Him!  May you live like it's that future time wearing a white robe showing how your dependance for Christ's sacrifice as purified you.