This was a surprise, and a sad one. Michael Nazir-Ali is one of the very few English Bishops who are courageous enough to stand up for the gospel, and speak out on ethical and social matters. His resignation is a loss to the C of E, and will mean even greater pressure on the few other Bishops who are bold and orthodox.
Newspapers and blogs are speculating about his reasons for going, but one can't help assume that the incredible pressure he must have endured from within the C of E and the house of Bishops, as well as from society (apparently he received death threats over his comments about the rise of Islam) were key. Yet I wonder whether, in the end, after much striving, he just felt he was fighting a loosing battle in terms of English Anglicanism. It is true to say that the Anglican hierachy in general are fiddling as Rome burns around them. And someone of Nazir-Ali's passion and integrity, would only persevere in seeking to reverse this, to the extent that he felt (humanely speaking) this may have some impact. Increasingly realising it doesn't, perhaps he just felt before God that he should spend his gifts elsewhere, where greater good could be done for the Kingdom. And so his apparent desire to work with the persecuted church.
In short, his resignation asks the question: "To what extent is it worth fighting to save the Church of England or reverse the meltdown in our society?" or "Whould our time and efforts be better poured out in greater evangelism or other noble work?" In truth, it is evangelism that has historically been the only real changer of society, as hearts have been regenerated and lives transformed by the living God. Consider the 18th century revivals.