Watchtower answer:
The 144,000 are the only ones who will go to heaven. They are all the anointed, born-again ones starting with Jesus Christ at His baptism until 1935. This is the year when Judge Rutherford proclaimed that he received his “revelation” that the “heavenly door” was closing and that an earthly hope was then being opened. This “new door” was for those who would become the “friends of Jehovah” by submitting to the “slave class.” They are also called “the 144,000,” the “children of God,” the “little flock,” the “spiritual seed of Abraham,” the “anointed ones,” and “kings and priest unto God.”
Bible answer:
The 144,000 of Revelation chapter 7 are identified in chapters 6 and 7 as those who will have been martyred during the Great Tribulation period (an event that is future) because they will have endured until the end and refused to take the mark of the beast. They are given white robes as a sign of their salvation. They are also numbered with the great crowd, and both the 144,000 and the great crowd are before the throne in heaven. The 144,000 will also rule with Christ on the earth during His literal, 1000-year kingdom reign. These verses in their context make these points explicit:
Rev 5:10 And hast made us unto our God kings and priests: and we shall reign on the earth. Note: See KIT p. 1090: upon the earth.
Rev 7:13-15 And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they? 14 And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb. 15 Therefore are they before the throne of God, and serve him day and night in his temple: and he that sitteth on the throne shall dwell among them.
Rev 6:9-11 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: 10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? 11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
Note: The Great Tribulation described in Revelation chapters 6-18 is a future event. This is the most obvious point that refutes all of what the Watchtower teaches about the 144,000. The 144,000 of Revelation chapter 7 are numbered with the Great Tribulation saints who will be put to death for their faithful witness affirming that Jesus is the Christ, the Messiah.