Adherents of the Christian Congregation of Jehovah's Witnesses are Christians.
A few self-described "Christians" have claimed that the title applies uniquely to those who believe precisely as they do. They apparently take for themselves a role which properly belongs to Christ himself.
.. ..(Matthew 7:21-23) Not everyone saying to me, ‘Lord, Lord,’ will enter into the kingdom of the heavens, but the one doing the will of my Father who is in the heavens will. Many will say to me in that day, ‘Lord, Lord, did we not prophesy in your name, ...and perform many powerful works in your name?’ And yet then I will confess to them: I never knew you! Get away from me, you workers of lawlessness.
Ironically, the bible most closely associates being "Christian" with preaching about Christ and Christ's teachings. Review all three times the bible uses the term "Christian" and note that the context connects the term with:
"declaring the good news"
'teaching quite a crowd'
'open eyes, turn from dark to light'
"uttering sayings of truth"
"keep on glorifying"
.. ..(Acts 11:20-26) [The early disciples of Jesus] began talking to the Greek-speaking people, declaring the good news of the Lord Jesus... and taught quite a crowd, and it was first in Antioch that the disciples were by divine providence called Christians.
.. ..(Acts 26:17-28) [Jesus said to Paul] I am sending you, to open their eyes, to turn them from darkness to light and from the authority of Satan to God... Paul said: “I am not going mad, Your Excellency Festus, but I am uttering sayings of truth and of soundness of mind. ...Do you, King Agrippa, believe the Prophets? I know you believe.” But Agrippa said to Paul: “In a short time you would persuade me to become a Christian.”
.. ..(1 Peter 4:14-16) If you are being reproached for the name of Christ, you are happy... But if he suffers as a Christian, let him not feel shame, but let him keep on glorifying God in this name
So why do anti-Witnesses try to hijack the term "Christian" and hide its Scriptural implications? Because anti-Witnesses recognize that it is the preaching work that makes it clear that the relatively small religion of Jehovah's Witnesses are by far the most prominent followers of Christ:
.. ..(Matthew 28:19,20) Go therefore and make disciples of people of all the nations, baptizing them in the name of the Father and of the Son and of the holy spirit, teaching them to observe all the things I have commanded
Technically, some Mormons (such as fundamentalists) are not members of the LDS Church.