At the most fundamental level, Roman Catholics subscribe to the Nicene Creed, Jehovah's Witnesses do not. (JWs would call the Creed itself heresy, so this is not a slam.) This means that unlike 98% of the world's Christians, Jehovah's Witnesses deny such things as:
- The Trinity
- The Divinity of Jesus
- The Nature of Salvation
- The Communion of Saints / The Role of the Church
and many, many more things.
Additionally, the Jehovah's Witness subscribe only to the New World Translation of the 66 books Protestants put in the canon. This translation is considered faulty by most Nicene Christians for translations such as:
- John 1:1,
In the Beginning was the Word, and the Word was with God, and the Word was a god
This implies that God is one god amongst several to Nicene Christians - something that Nicene Christians would reject.
- John 10
I and the Father are One in Unity
This addition is an attempt to reject one of the most obvious prooftexts of the Trinity. As Nicea is explicitly Trinitarian and this is not, it likewise excludes JWs from that category.
When most people think of "Christianity" they are thinking of Nicene Christianity. It is entirely fair to say then that Roman Catholics are what most people consider "Christian" and Jehovah's Witness are not.