Many Christians who are even worldly or new to the faith know that the JW's, and Mormons, etcetera are not saved. The issue comes down to knowing about the right Scripture verses.
God reveals Himself as triune or as a Trinity.
The King James Bible uses the word
Godhead (Meaning Trinity).
1 John 5:7 (KJB) says “For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.”
They don't believe that verse; Nor do they believe the three occurrences of the word Godhead means Trinity (Which fits the context). Jesus is GOD, and they deny this obvious fact in Scripture. They deny the personhood and deity of the Holy Spirit, as well. The Bible makes it clear you cannot worship false gods. To not worship the Trinity or the Godhead as revealed in Scripture is to worship a false god of one's own imagining. They also deny the bodily resurrection. Read 1 Corinthians 15 again. You cannot deny the bodily resurrection and be saved according to this chapter.
Yes, the RCC (Roman Catholic Church) has many errors. So does the JW church. Really. Just Google the false beliefs of JW's and read about 25 to 30 articles or so why Christians regard them as non-Christian. They don't worship the same God as Christians do. Christians believe in the Trinity. JW's don't believe in the Trinity. JW's deny bodily resurrection. So these are not small or light matters that they are rejecting. Do you have a friend or family member who is JW? If so, you should not let personal feelings overrule what the Bible says on this matter, sister. It's clearly obvious JW's are not Christian. This shouldn't really be even debatable unless you believe those who reject of the Trinity and the bodily resurrection are saved. If so... you need some solid biblical evidence to prove your case.