We must not pick up the book of Revelation without understanding that it is written in an apocalyptic style, a style of literature that employs symbols via images, creatures, numbers and colors. This is not a book that one can read as a carpenter does a blueprint or a cook follows a recipe. The numbers, for example, do not necessarily imply a literal equivalent, but are symbolic of a larger and deeper truth. The number 144,000 was used as a huge number, with a multiple of 12 (which itself is a much-used symbolic number in the book of Revelation) to depict an imprecise but huge group of people who, by God’s grace, inhabit the new, spiritual Jerusalem. They receive spiritual healing and are sealed from further harm by his own blood.
I explain this passage in greater detail in my book “Revelation Revolution”
— Greg Albrecht