Many Protestant and Evangelical churches and Christians view freemasonry as incompatible with Christianity and teach that a Christian should not be a member of the group. What is the basis, biblical or otherwise, for this position?
I want to preface this by saying that I have never been a Freemason, and this answer, necessarily will include items that are likely to be disputed by those within Freemasonry. This can't be avoided if I'm to answer the actual question. The question is "Why do Evangelicals consider it to be incompatible". It's not asking if it really is, just why they think it is.
In order to answer that, I need to post the things that those who oppose Freemasonry believe. I have absolutely no idea if those things are true or not, and I'm not here to claim they are or aren't. I'm just repeating what's in the literature and what's believed of Freemasonry by Christians.
I'm going to use, as my primary resource, a site that's been put up by someone claiming to be an ex-Mason. It contains much of the same in formation that can be found in other literature. I've read several books on "cults" and Freemasonry is listed in several of them. For example, while it's not listed as a cult in this book, the book What's the Difference lists Freemasonry in a section called "Nothing New Under the Sun... Eleven more viewpoints that undermine, challenge, or attack Biblical Christianity". At any rate, the claims on this site are compatible with most of what I've said.
With all of that out of the way, here are several points about Freemasonry that are incompatible with Christianity:
Another site lists other reasons, again, which I've heard before, but can't personally verify. Again, I'm just answering why some Christians are opposed to Freemasonry.
Freemasonry is a fraternity that does not refuse people due to their specific religious preferences. But there are some exceptions. For an example, an atheist is strictly and universally (under the grand lodge of England) not permitted to join. Because we allow multiple faiths, some people take issue with the fraternity. Such as Christians are taught to not to yoke with unbelievers
2 Corinthians 6:14 NIV
Because Freemasonry does not support a single specific religion but requests that it members subscribe to a religion and as mentioned "yoking", some people such as the Sovereign Pontiff. Maximillian Kolbe believe that implies that they are against Christianity.
Christians who do not believe you should be friends with non-Christians would have a problem with the fraternity.
Freemason takes obligations or makes promises which some Christians dislike. So for example, I promise to not allow an atheist to join. Some Christians feel that you should not make promises like that to yourself or in front of God.