Revelation 7:4 And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel.
To the Little Remnant Scattered Abroad 04-06-1846 The living saints, 144,000 in number, knew and understood the voice, while the wicked thought it was thunder and an earthquake.
The 144,000 are the living ones that go to heaven. Yes, i know that there are some SOP quotes that seem to suggest that some in those days were sealed, but we see that God has spared us more time, so they were laid to rest in the grave. All those who died in faith under the 3rd Angel's Message, which started in 1844, will be raised in a special resurrection before the general "1st Resurrection".
I believe the great white multitude which no man can number are those sealed from ages past, not now alive. The 144,000 are the firstfruits of the earth.
This implies their being alive at or around the time of rapture.