That’s an expression meaning to denigrate two separate, different things as though they were one and the same. It is an insidious logical fallacy, and unfortunately one that slips under many people’s radars.
Two things that are so tarred by many Christians are people who are striving to create a New World Order and people who say they are part of the the New Age Movement. Some Christians are trying to say that those two movements are one and the same. There may be some similarities between the two, and there are those who belong to the first, but claim to belong to the second, thus muddying the waters further. But the two are not one and the same.
Also, the NAM covers a huge territory, and yes, there may be some overlap among individuals, but to damn the entire NAM because there are some who are misguided or have bad intentions is wrong. You might as well damn the whole of humanity, because none of us, no matter who we are or how Christian and God-fearing and Bible-thumping we may be, are in perfect enough communion with God to truly know what’s going on at a global level.
Of course, what is really going on with this tarring is that those Christians who are lumping in the two movements together think that they are right and everyone else is wrong. There are a few things in particular about what they are saying that strike me as not only wrong, but harmfully wrong; things that are bound to cause division between people who call themselves Christians and New Age people who are just as Christian (or more so than some) in their behavior, morals, ethics, and ideals. (Defining being Christian as a way of life that includes how you think about and treat other people based on what Jesus—not Paul, but Jesus—said.)