It was speaking after divisions emerged in the Labour Party's shadow cabinet about its position on whether to bomb Islamic State (IS) in the country.
Jeremy Corbyn has remained opposed to airstrikes in Syria, saying that it will increase the threat of terrorist attacks against the UK.
He advocates cutting Islamic State funding instead.
However the Shadow Foreign Secretary Hilary Benn has said he supports extending the bombing campaign against the militant group from Iraq into Syria, although he will not resign over the issue if the party decides to oppose airstrikes.
The Deputy Leader Tom Watson has said Prime Minister David Cameron made a "compelling argument" for airstrikes against the jihadis in Syria.
Britain is already bombing IS targets in neighbouring Iraq, with analysts disagreeing on the extent to which the two situations are different.
Labour MPs are meeting on Monday to discuss the issue further. The Labour Party could take a policy of support or opposition, or allow MPs to vote with their conscience.
Stephen Beer, from Christians on the Left, told Premier: "I certainly think we should be having the conversation and I think we should be doing it in a way that gets around this view that you're either for or against military action, as I think it's more complicated than that.
"You've just got to take it on a case by case basis, but you've got to have some degree of judging it. The reason we always say these things are a last resort and should be is because the term 'a necessary evil' says it all really.
"There are certain things that must be [the case] for a just cause. It must be done with the right intention, it must be a last resort. There must be some sense of an idea of what will happen afterwards, as well as what we're actually trying to achieve."
Just war theory has been a part of Christian thought for centuries. Theologian Thomas Aquinas gave three conditions for a just war:
1) War must be waged by a properly instituted authority such as the state;
2) War must occur for a good and just purpose rather than for self-gain;
3) Peace must be a central motive even in the midst of violence.
Christians disagree on which wars in history were justified and which were not, and the extent to which war can be justified at all.
Listen to Premier's Aaron James speaking to Stephen Beer here:
