Is a Muslim's residence in a land of disbelief – for the purpose of completing education, acquiring Islamic knowledge, and calling to Allah – permissible in Islamic law?

You are permitted to remain in the country you are in if you are safe from temptation for yourself, and you are able to perform the rituals of your religion. If, in addition to that, you strive to offer sincere advice and invite people to Allah Almighty, that would be even better.