I think this is a question of terminology as well as being a question of theology. I’ll try to address both.
The vast majority of branches of Christianity will divide the world up into groups which look roughly like this:
- Us (the people who agree with us theologically, and associate with us);
- People we disagree with on minor matters;
- People we disagree with on fairly major matters, but whom we still accept as Christians;
- People who are completely wrong (“Heretics” / “people who claim to be Christian, but aren’t”);
- Non-Christians / members of other religions (“people who don’t even claim to be Christian”);
(Many won’t see any need to distinguish between those last two groups, of course.)
One important difference between the Witnesses and mainstream Christianity is the sizes and memberships of the various groups above. Jehovah’s Witnesses are very united, and believe that acting as a group is important. As such, they do not accept the Christianity of anyone outside their faith. At least, not in modern times. They accept that others will have glimpses of the truth, and have said complimentary things about Luther, Calvin, and other luminaries of the Reformation, but they think that since their own organization started God has uniquely blessed it.
As such, the second group (“people we disagree with on minor matters”) is pretty much non-existent to the Witnesses, unless you count the very small differences within the faith. (As I said, the Witnesses are a very united group, but the leadership don’t pin down every single theological question; just most of them. Witnesses can differ from each other on minor matters, but are cautioned not to hold any such belief as an actual teaching, just as a personal hypothesis. That’s acceptable.)
The third group (“other Christians”) is also pretty much non-existent in the modern day. Jehovah’s Witnesses do not accept anyone other than themselves as Christians. (To be fair, other Christian groups usually return the favour.)
While you might expect to fall into that third group as far as the Witnesses are concerned, in fact you fall into the fourth: you would be seen as a member of “Christendom” (this is another word which has a different meaning in Witness theology to its normal geopolitical meaning). “Christendom” (false Christians), non-Christians, and atheists are all lumped together as “non-believers”.
Note that if you were to pursue a romantic relationship with a Witness, this would be regarded as dating outside the faith, becoming “unequally yoked” with an unbeliever, and strongly disapproved of by his fellow Witnesses.
- Theology: Witnesses regard modern-day non-Witnesses as non-Christian.
- Terminology: Witnesses lump all non-Witnesses together as “non-believers”, whatever they actually believe in.