Does this Hadith: "Every Muslim is forbidden to another Muslim, brothers supporting each other. Allah Almighty will not accept any deed from a polytheist after he has embraced Islam, nor will He accept until he separates from the polytheists and joins the Muslims," apply to someone who was in a state of Jaahiliyyah (ignorance before Islam) and committed to Islam while residing in lands of disbelief?

The hadith mentioned in the question is authentic, and its meaning is that a Muslim must separate from the polytheists by migrating from their lands if unable to establish the religion there, or by separating from them in dress and customs. The hadith does not apply to one who is secure in his religion, himself, and his wealth, and is able to perform his religious rituals. The evidence for this is the Prophet's (peace be upon him) saying: "There is no Hijra (migration) after the Conquest, but there is jihad and intention," and his saying to a Bedouin who asked him about Hijra: "Work from beyond the seas, for Allah will not neglect anything of your work."