Syncretism and Freemasonry
Syncretism
What is syncretism? Simply put, it's any religious view that equates multiple (or all) religious dogmas about deity. It also usually implies some sort of synthesis of tradition and ritual. Sometimes syncretism which involves all notions of deity is called "omnism" (e.g. the claim that every religion is worshiping the same God, just with different names, aspects, avatars, etc.) A radical omnist would say that it's impossible to create a "false religion" because any attempt to formulate something called a religion would be cast in terms of your latent understanding of the divine.
So those are the various shades and subtleties of syncretism... Why would people think that Freemasonry is syncretic? Because we discuss deity and have rituals that have religious elements, and yet we accept members of all of the major religions (and most of the minor ones). So, to the casual observer, this looks like syncretism.
Why isn't this syncretism, then?
The crucial reason that this is not syncretism is one of direction. Freemasonry does not say, "here is a God which fits everyone's ideas, and you should worship this." It's in the other direction. We say, "you have a conception of God, let's talk about what the implications of that might be." At no point do we need to know or seek to modify that belief, nor syncretize it with some other belief. We simply follow a rational line of thought that stems from the core assertion of deity. No Freemason needs to accept that this line of thought is grounded in a true understanding of God on the part of any other Freemason, they need only pursue that understanding through the lens of their own religious beliefs.
When we talk about God in Freemasonry, we're not talking about Krishna or Allah or Jesus. We're instead talking indirectly. Those parts of our traditions that mention God only make sense through the individual Brother's lens of religious belief.
What about the rituals?
It's easy to think that the rituals of Freemasonry are syncretic; in fact, far easier than it is to think that the idea of God in Freemasonry is. There are Christian elements, Jewish elements and more abstract elements. Isn't this syncretism? It's a fine line, but I'd argue not. The ritual syncretism of Freemasonry melts away when you consider that all of the elements that are "borrowed" are being used allegorically to refer to abstract ideas, not as a form of direct worship. Indeed, Freemasonry tells us to seek out the Worship of deity through our own religion.
But the converse...
It is, of course, possible to by a syncretist and a Freemason, but it is equally possible to be a non-syncretist and a Freemason. Ultimately, sitting in Lodge with a Brother of another faith does not require you to accept his religion as valid, only to respect him and not to bring secular religion into an open Lodge. This is not just a nice thing to do, it can be absolutely crucial! Freemasons might be of religions which are openly hostile to each other, and the Lodge neither asks that they relinquish that animosity, nor that one or the other accept their opposite number's view. They must simply leave their sectarian religion at the door and sit down as two men trying to be better for the duration of the meeting.
The same is true of politics. We're not saying that liberals, conservatives, centrists, anarchists or anyone of any other political stripe need to agree on political matters, just that they need to sit down and be Brothers for a little while.
So the short answer is that Freemasonry is not a form of syncretism because Freemasonry does not have any conception of God. It symbolically and indirectly uses the individual Brother's idea of God to teach certain moral and philosophical lessons, but never tells you what that belief should be.