Upvote:6
This is from the perspective of Jehovah's Witnesses.
The how and when of the 144,000 being gathered is answered by Jesus' illustration of the Wheat and the Weeds described at Matthew 13:24-30, 36-43:
24 He presented another illustration to them, saying: “The Kingdom of the heavens may be likened to a man who sowed fine seed in his field. 25 While men were sleeping, his enemy came and oversowed weeds in among the wheat and left. 26 When the stalk sprouted and produced fruit, then the weeds also appeared. 27 So the slaves of the master of the house came and said to him, ‘Master, did you not sow fine seed in your field? How, then, does it have weeds?’ 28 He said to them, ‘An enemy, a man, did this.’ The slaves said to him, ‘Do you want us, then, to go out and collect them?’ 29 He said, ‘No, for fear that while collecting the weeds, you uproot the wheat with them. 30 Let both grow together until the harvest, and in the harvest season, I will tell the reapers: First collect the weeds and bind them in bundles to burn them up; then gather the wheat into my storehouse.’”
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36 Then after dismissing the crowds, he went into the house. His disciples came to him and said: “Explain to us the illustration of the weeds in the field.” 37 In response he said: “The sower of the fine seed is the Son of man; 38 the field is the world. As for the fine seed, these are the sons of the Kingdom, but the weeds are the sons of the wicked one, 39 and the enemy who sowed them is the Devil. The harvest is a conclusion of a system of things, and the reapers are angels. 40 Therefore, just as the weeds are collected and burned with fire, so it will be in the conclusion of the system of things. 41 The Son of man will send his angels, and they will collect out from his Kingdom all things that cause stumbling and people who practice lawlessness, 42 and they will pitch them into the fiery furnace. There is where their weeping and the gnashing of their teeth will be. 43 At that time the righteous ones will shine as brightly as the sun in the Kingdom of their Father. Let the one who has ears listen.
Here's an info-graphic from a 2013 Watchtower article - “Look! I Am With You All the Days”
Angels will harvest (or "reap") the wheat class of anointed Christians, bringing them into Jesus' storehouses. These storehouses represent the restored Christian congregation. They are then brought to heaven in these ways:
The harvest season “is a conclusion of a system of things,” which began in 1914. Anointed Christians have been gathered into the storehouses of the congregation since 1919, and the exact times of the Great Tribulation and Armageddon are at some unknown time in the future.