"Physical coercion" refers to using physical force or violence to make someone do something against their will. It involves using aggression or harm to make someone comply with demands or instructions. Full definition
And in the same paragraph: «It is really very difficult for people — including large associations of people and huge corporations — to do anything very bad, for very long, when they are not buttressed by the threat of physical coercion.» (firstthings.com)
According to his biographer Adrian Murdoch, Julian «marginalized Christianity to the point where it could potentially have vanished within a generation or two, and without the need for physical coercion.» (firstthings.com)
He emphasizes freedom of moral choice over physical coercion to do what's morally right. (religion.blogs.cnn.com)