What does "Once in the grand whole of the Mighty One, so she must continue an integral part." mean?

What does "Once in the grand whole of the Mighty One, so she must continue an integral part." mean? and does "self-hood" here mean "selfishness"?

Unless at the coming crisis the Great Power intervenes, that is, the Grand Operating Power of which I have spoken before, and in calm dignity flows forth and issues the mandate—Peace, be still!—the prophecy of some, that England shall sink in the depths for ever, will be fulfilled. Like the specific atoms of life who compose the State called England, who must sink for a time in order that they may rise again, even so must the Nation sink, and that to a great depth for a season; because she is immersed in the love of what is false, and has not yet acquired the intelligence that will act as a powerful lever to raise her up to her own dignity. Will she, like a drowning man going down for the third and last time, go down and be lost for ever? Once in the grand whole of the Mighty One, so she must continue an integral part. There is a kindly hand that will be stretched forth to save her, and bear her up from the billows of the self-hood that would otherwise engulf her. With an energy that is irrepressible, that power says – England once, England forever! But not in the same state will that continuance be. She must and will sink the lower, in order that she may rise the higher. The how, why, and in what manner, and by what treatment we shall use to bring about her safety and serenity, I shall speak of further on; but, here I affirm, that in order to save her, England must be drained of her best blood.

"The History of Spiritualism," by Sir Arthur Conan Doyle