"Imagine no heaven" by Salman Rushdie is about individuals struggling with a complex sets of external forces that regulate and define their lives. The external forces may be represented by laws, religion, tradition, and society. What are these external forces in Rushdie's work?

In "Imagine no heaven," Rushdie sees orthodox religion as an external force that regulates an individual's life.
Rushdie's letter is written to the six billionth person born on the planet. He offers advice on two critical levels to this little person: "How did we get here? And, now that we are here, how shall we live?" In answering these questions, Rushdie suggests that dogmatic religious structures represent an external force that regulates and defines people's lives.
Rushdie lays out his case to the six billionth person on the planet in several ways. Rushdie believes that religious dogma seeks to control people's lives because it requires "much more" of the individual. Rushdie argues that "rituals of worship that grow up around" people become a part of their lives. He tells the six billionth person that religion will eventually become "the heart of your culture, even of your individual identity." This construction suggests that religion all over the world has a profound effect on how people see and define themselves.
Religion's pretense for abuse is another way Rushdie sees its controlling nature. Rushdie argues that while its message might be universal harmony, people who hold a zealous view towards religion might embrace it as a form of "public oppression." They persecute others as they see themselves as "charioteers of the gods." Rushdie feels that these people do not have a problem with religion being the pretext for so much human suffering: "In the opinion of religious people, however, the private comfort that religion brings more than compensates for the evil done in its name." The wars that religion fuels represent regulation of human action:

The real wars of religion are the wars religions unleash against ordinary citizens within their "sphere of influence". They are wars of the godly against the largely defenceless—American fundamentalists against pro-choice doctors, Iranian mullahs against their country's Jewish minority, Hindu fundamentalists in Bombay against that city's increasingly fearful Muslims.

The destructive capacity of religion is where Rushdie's fears about its effect on individual identity are most pronounced.
Rather than present an entirely dark portrait of humanity being regulated by religion, Rushdie suggests that there can be a way for individuals, particularly the six billionth person born, to reassert control over their lives. Rushdie feels that openly questioning dogmatic notions of the good is one response to a potentially oppressive force: "The victors in that war must not be the closed-minded, marching into battle with, as ever, God on their side. To choose unbelief is to choose mind over dogma, to trust in our humanity instead of all these dangerous divinities." Rushdie believes that embracing thought is a viable response to religion's potential for control.
Rushdie wants people to embrace their ability to think. He believes that making conscious choices that represent complexity and nuance as opposed to reductive simplicity is a way for people to resist the controlling force that religion could be:

 Only you can decide if you want to be handed down the law by priests, and accept that good and evil are somehow external to ourselves. To my mind religion, even at its most sophisticated, essentially infantilises our ethical selves by setting infallible moral Arbiters and irredeemably immoral Tempters above us: the eternal parents, good and bad, light and dark, of the supernatural realm.

As Rushdie closes his letter, he believes that individuals do not have to succumb to the pressures of religion's propensity for control. He believes that reveling in our ability to think is something that "all six billion of us could do for ourselves." He believes that thinking for oneself is a way for individuals to participate in "the revolution in which each of us could play our small, six-billionth part: once and for all we could refuse to allow priests, and the fictions on whose behalf they claim to speak, to be the policemen of our liberties and behaviour." To see the world as "undogmatised and plain" is a way to repel the controlling aspect of religion. Rushdie believes this is the way out of the regulation and control intrinsic to religion.