The announcement by Boris Johnson that he's backing the Brexit side in the forthcoming EU referendum vote is a blow to the Prime Minister. Cameron engaged in marathon negotiation sessions to cajole and browbeat the other EU countries into supporting curbs on benefits to migrant workers and their families. Not well enough, said Boris; he'll vote Leave.

Fair enough, it might be thought. Boris knows Europe and the EU – and they are definitely not the same thing – very well, having served as the Telegraph's Brussels correspondent. He's personally a Europhile, with a far greater knowledge of its ancient ways than most of his colleagues and rivals. He has a right to an opinion.

In spite of his intervention having goosed the referendum into life, however, there's much that isn't yet clear – and may not be by June 23. Among these very practical questions are:

1. Will Britain be better off financially if we leave?

No one really knows and there are reasonable arguments on both sides. FTSE chiefs say leaving would put the economy at risk. Pro Brexiters say we can negotiate trade deals like Norway and Iceland do and be perfectly happy.

2. What about Scotland?

Scotland voted en masse for the SNP, which is pro-EU. If Britain votes to leave, it's argued that it would revitalise the nationalist cause and provoke another independence referendum. Possibly, though Scottish voters might vote to leave the EU too.

3. Would Brexit help curb migration?

Probably, though this comes at a cost; employers like to be able to hire whomever they want, and a liberal migration policy will be essential for growth.

4. Would it make the country safer?

Former Tory leader Iain Duncan Smith says remaining in the EU makes hard to fight terrorism; Europol director Rob Wainwright says it makes it easier. Take your pick. 

We may be wiser by June 23, as arguments are tested in public debate. However, there's more to it than numbers and political calculations. Christians ought to be profoundly engaged in the EU referendum because the result will affect so many lives. We ought, though, to be able to lay down some principles, make some connections and challenge some assumptions. So here are a few.

1. Too much of the Vote Leave argument is based on a narrow nationalism.

Not everyone who believes Britain would be better off outside the EU is a narrow nationalist; I don't think Boris is. But that is the tone much of the argument is taking. It's driven by the same sort of rhetoric and assumptions that has driven the rise of Donald Trump and Ted Cruz in the US. That alone ought to make us even more rigorous in testing the leavers' arguments.

2. Christianity is fundamentally about collaboration, not competition.

Much of the argument is not even about whether Britain can prosper outside the EU; it's about sovereignty and whether Britain should allow a certain amount of "pooling" of sovereignty for the greater good of Europe as a whole. We can argue about whether it works, in terms of the influence Britain has, but Christians cannot argue, as Christians, that pooled sovereignty is wrong in principle. The question is not just what's best for Britain, but what's best for Europe.

3. Peace is worth preserving.

The origin of the EU lies in the years immediately after the Second World War, when the European Coal and Steel Community was founded. It was originally proposed by French foreign minister Robert Schuman in 1950 as a way of making another war between France and Germany "not only unthinkable but materially impossible". It's arguably the European project, which has made European countries accountable to each other and dependent on each other, that has kept the peace.

Would Britain be "better off" outside the EU? It's impossible to say. But that is not, perhaps, the right question. Arguably a better one is, "Would the nations of the EU be more prosperous, more just and more secure with Britain inside it?"

Follow Mark Woods on Twitter: @RevMarkWoods