So what exactly is the problem with Masonry?
I'm not an expert on Masonry, but my own opinion is that Christians should not be involved in any type of "secret society" that involves oaths, pledges, swearings, etc. The reason is because I believe such organizations and groups are essentially counterfeit replacements of The Church. Such groups usually promise the hope of unity, fraternity, brotherhood, the accomplishing of certain goals (which are often quite noble goals), solidarity, etc. In other words, they offer things that can only truly and authentically be found in and through the Church. Also, such groups can often instill a sense of superiority and foster prejudice. "Our fraternity is the best." "We are special because we are part of this select, secret organization." etc.
Certainly, some secret organizations are more nefarious than others. Skull and Bones and Free Masonry are probably more frought with subversive doctrines and evil ideas than the average college fraternity. But as a personal rule, I eschew any involvement in groups or organizations that involve secret pledges and such. If we are part of The Church, then what more do we need to be a part of? What do these organizations and groups offer us that we cannot find in and through the Church? Whatever they offer that the Church does not is at best superfluous to our salvation, and at worst detrimental to it.