Yes - the problem is the secrecy and the ritual - that evokes all kinds of suppositions.
But I would agree with Forever Settled that they need to hear the gospel.
The problem being that in the lodges with a bent towards Christianity its a hard sell as they don't think its necessary to renounce or quit the lodge or temple.
I know of one Baptist church in Maine that just about the entire male membership are Masons (at least they were about 30 years ago).
The problem comes in when people think that by virtue of one being involved with Freemasons or Boy Scouts that they cannot be a true Christian (or even a faithful Christian). I’ve been told by several people that I forfeited my faith when I joined the Army and took an unholy oath to our nation. I take it with a grain of salt (basically I don’t let other people’s opinions bother me).
The reason I say it is a problem is that there is the credibility of two witnesses at stake – the Freemason (or Scout) and the believer convicted that it is wrong.
The Freemason could cause a brother to stumble because that person truly believes that the mason worships Satan while attending church claiming to be a Christian. That person is convinced of the things he or she has read, and they apply this to all Masons (they see books like Morals and Dogma as expressing what the organization is about).
On the other hand, what if the other person is talking to a Freemason who is not a Christian and whose experience is completely different to what the Christian is describing? We have to be careful when dealing with people that we do not accidentally invalidate our testimony by speaking to what may not be accurate across the board.
I say this as someone who has been a Mason, a “past-master” (I presided over a lodge and gave the “degrees”), a York Rite Mason (Royal Arch, Cryptic, and Templar), and Scottish Rite (32 degree). I do not believe a Christian should be a Mason, and that is why I left the organization. I believe it is Gnostic in nature (or tries to be). The primary reason I left the organization, however, was so as not to be a stumbling block to a brother. People believe all kinds of things (even things that have been proven incorrect).
I believe the only danger in people believing the demonic myths surrounding the organization is that they ruin any opportunity of reaching a non-Christian Mason with the truth. It's not worth confronting the zealous believer because their intentions are well and they will typically not be persuaded that they are damaging their testimony by overreaching. From experience, if I were to tell you they do not worship Baal then I would be accused of being brainwashed or being pro-Freemason (I am neither, BTW). So when it comes to the charge of worshiping Satan, leave it at "that may be true of some, but that was not my experience". And I offer to share my experience.
We do have to remember that it is not necessary to quit the Freemasons in order to be saved. And many Christians joined the Masons because their churches did not "walk the walk" (you may be surprised how often this happens) and they look for a way to actively live out their faith.