Jehovah's Witnesses (JWs) claim to regard the Bible as the absolute Word of God and to base all their beliefs on it. In fact, the teachings of JWs are contrary to the Bible.
The Bible. JWs use a doctored version of the Bible called the New World Translation (NWT). The JW leaders who produced the NWT were not biblical scholars, and it shows. The most obvious difference between the NWT and other Bibles is its use of "Jehovah" in the NT. JWs claim that the NT originally used the Hebrew name YHWH (translated "Jehovah" or "Yahweh") and that apostate scribes put "Lord" (Gk kurios) in its place. There is no historical or manuscript evidence for this claim.
The Father, Son, and Holy Spirit. JWs teach that the Father alone is Jehovah, the almighty God; that the Son, Jesus Christ, is "a god" (their translation of Jn 1:1) inferior to the Father; and that the "holy spirit" is an impersonal force emanating from God. The Bible, on the other hand, teaches that the Father, Son, and Holy Spirit are each God (Jn 1:1; 17:3; 20:28; Ac 5:3-4; 2 Co 3:17-18; Ti 2:13). The Son made every thing (Heb 1:10-12) and is to be honored as God (Jn 5:23; Heb 1:6; Rv 5:13). The Holy Spirit is a person, called the "Comforter" or "Helper" (Gk parakletos); He teaches, speaks, and bears witness to Jesus (Jn 14:16,26; 15:26-27; 1G:33-14).
Death, the soul, and eternal punishment. According to JWs, when unsaved human beings die, they cease to exist. There is no intermediate state of the dead and no eternal punishment for the wicked (who are annihilated instead). The Bible, on the other hand, teaches that human beings exist after their deaths as spirits awaiting the resurrection and final judgment (Lk 16:19-31; 23:43; Heb 12:9,23; Rv 6:9-11). (The NWT mistranslates Lk 23:43 and the Hebrew texts to avoid this implication.) The wicked will suffer eternal punishment (Mt 25:46; Rv 14:9-11; 20:10).
Jesus' resurrection and return. JWs believe that God "raised" Jesus from the dead as an angelic spirit, with a so-called spirit body. They deny that He will return visibly and personally to earth. Scripture, however, teaches that Jesus rose with the same physical body with which He died, though glorified and immortal, and that His body possessed flesh and bones, hands and feet, and even marks of His crucifixion (Lk 23:49; Jn 2:19-22; 10:17-18; 20:20,25; Ac 2:24-32). Though He is the second person of the Godhead, Jesus is also a glorified man (Ac 17:31; 1 Co 15:47; 1 Tm 2:5) and He will return personally and bodily to the earth (Ac 1:9-11; 3:19-21; 1 Th 4:16: Heb 9:26-28).
Salvation. JWs view Jesus' death as providing a "corresponding ransom," releasing all people in principle from the condemnation due to Adam's sin. However, to enjoy everlasting life, JWs believe they must not only accept Christ's ransom but also prove themselves worthy by their works. The Bible's teaching is quite different. Christians are saved by God's grace alone, through faith in Christ, and our good works are the fruit of salvation, not the prerequisite for it (Rm 3:21-28; 5:1-11; Eph 2:8-10; Ti 3:4-8).
Extracted from the Apologetics Study Bible.