As Antoninus, my city and country is Rome: as a human being, it is the world. So what benefits these two cities is my only good.
In this passage, Marcus contemplates his relationship with god or the gods. He does not believe the gods could have mal-intent towards him. It is difficult for him to think that the gods are set on harming him or others intentionally. Rather, he takes the approach that they, indeed, have thought out the good for humans. But even if the gods did not exist, he still can create his own opinion about what is best. And his definition of best is to act according to his rational and social nature - which is nothing other than serving others in the spirit of the common good.
What is good for the city and Rome and for the world, is also good for him.
These thoughts, especially when considered in the context of a person experiencing events that may be painful or repulsive, help him and us to accept what the gods or fate dishes out to us.
In my opinion, this has got to be the ultimate test: to accept and love whatever life sends your way. I've observed that some people really excel at handling all the pressures and obstacles of life. For others, it takes longer to figure things out. But again, I've come to the conclusion, one of the ultimate tests of life is being able to accept and love some really, really bitter pills. This is why I put Nietzsche's words over a picture of a family struggling to survive The Great Depression. To be able to accept that fate and love it? Wow - a breathtaking challenge to say the least.
(see also Citadel p. 29, 43, 154, 159, 211, 268)