What is a central idea in Of Mice and Men by John Steinbeck?

One universal idea that prevails throughout the narrative of Of Mice and Men is that of the strength that lies in the fraternity of men.

As one who believed in the ideology of socialism--perhaps even communism--John Steinbeck perceived the homelessness and alienation of the dispossessed men as a result of failed capitalism in the 1930's. The only strength and hope that these men could gain was in unifying with others. When some of the other workers at the ranch, such as Candy and Crooks, hear about George and Lennie's dream of owning a little farm of their own, it is only then that they begin to have some hope for the future by joining in this venture. 

When Curley, the son of the boss, comes into the bunkhouse in a bellicose manner time after time, the men must do their best to not antagonize him lest they lose their jobs. Then, when the amazingly strong Lennie crushes Curley's hand, they all feel empowered, knowing that Curley will stay away from the bunkhouse while his hand heals, and out of embarrassment he will not tell the truth of what has happened to this hand. 

After Candy loses his old dog, he is very forlorn and anxious about his job as swamper. However, he regains hope after he asks if he can be part of George and Lennie's dream of owning a farm.

"S'pose I went in with you guys. Tha's three hunderd an' fifty bucks I'd put in. I ain't much good, but I could cook and tend the chickens and hoe the garden some. How'd that be?"

George half-closed his eyes. "I gotta think about that...."
But, then, George calculates if he and Lennie save their wages at the end of the month, they would have one hundred dollars more. "I bet we could swing her for that." For, the first time, George ponders his and Lennie's dream as a real possibility because of another man's joining in with them.

Later, when Candy tells Crooks about the place they are going to have, Crooks asks if he can be a part of this dream, too.

"...If you...guys would want a hand to work for nothing--just his keep, why I'd come and lend a hand. I ain't so crippled I can't work...if I want to."

With the fraternity of the men, the dream of George and Lennie becomes a real possibility.

The renowned metaphysical poet John Donne wrote that "no man is an island, entire of itself"...men must join together in order to have strength, helping each other to have a life with some dignity. As Crooks has noted, a man needs someone else with him by whom he can measure himself.