What is the basis that many mainstream Christian groups justify labeling groups like the Mormons and Jehovah Witnesses as Non-Christians?

If there is no clear definition stemming from the Bible of what makes one a Christian, how does mainstream Christianity justify labeling groups like the Mormons and Jehovah Witnesses as Non Christians? Members from both groups seem to take a more active role in spreading Christ's message of salvation than do the most rooted mainstream Christian church members, based off of their missionary efforts even though some of their doctrines do not conform to the norms as held by most Christians. There is evidence enough to come to their doctrinal conclusions, even if we disagree with them. So is non normative theology enough for mainstream Christianity to label others as non-Christian?