Their sincerely held belief is that Jesus Christ is the only true God, the Way, the Truth, and the Life. An acceptance of Jesus as Lord, the Way to eternal salvation with God in Heaven.
Now these are very powerful beliefs, and we must acknowledge that they are entitled to have them.
These ex-Masons (and most probably they are); why do they become so anti?
Firstly guilt. I suspect that they cannot accept that they went into Freemasonry with their eyes open, and are looking to blame Freemasonry for "duping" them or "trapping" them in some way.
Blaming others for one's own "bad" decisions reflects guilt away from those who will not take personal responsibility for those decisions. "I was tricked!".
Once they reach this point, they want to ameliorate their guilt by "saving" others from being "tricked" by those evil tricksters, the Freemasons.
The problem, and it is a problem that we must face, is that "mainstream" Freemasonry was, and is essentially religious. I know that this may not be popular in some circles, but it is a fact.
Prior to the
Moderns era, the Church actively supported, indeed were the leaders of what was, Catholic Freemasonry. So what changed?
It is often said that Anderson de-Christianised Freemasonry. His intention however was to retain the essential religious tenets of Christianity, but allowing others to participate by utlilising the catch-all phrase "... a supreme being".
At that time there were no Buddhist or Muslim Freemasons. There were Catholics, Protestants and few Jewish Freemasons in England, and definitely no Athiests or "Religious Libertines".
The problem comes, not with what he did, but how he did it. Even at the time there was a massive anti reaction, even within his own Grand Lodge jurisdiction.
His concept of "... a supreme being" opened the doors to "mental reservation". Other faiths came along later, Muslims, Buddhists, Hindus, Sikhs.
Today, it seems, anything goes. "Conscience" is the new supreme being.
This is their problem. They see Freemasons, of many, all and any beliefs, meeting in an essentially religious context, in a Masonic temple, praying to God, swearing Oaths, invoking the word of God.
They see that as accepting, or at the very least acknowledging the claim of the validity of other religions, all religions, or even no religion at all "concience".
This is not only a failure, but an encouragement to others not to proselytise Christianity, the duty as they see it, of every true Christian.
The Christian God - yes: "... a supreme being" - no.
This is a part (a large part) of the great Masonic debate that has gone on since the early 1700's.
Remember, they have only experienced post Anderson Freemasonry and take their stand accordingly.
That IMHO is the why. What we do, how we deal with it, whether we should do or say anything, these are other questions.