The Muslims4Lent Twitter hashtag is getting quite a bit of traction.

It was started last year by two Muslims, Bassel Riche and Salmaa Elshanshory of Eid.Pray.Love, wanting to reach out to Christians and show solidarity with us by giving up things they liked doing or eating during Lent, just as we do.

That initiative inspired the #Christians4Ramadan movement, promoted among others by Julian Bond of the Christian Muslim Forum. And let's be honest: Ramadan is a good deal tougher than Lent. In the spiritual disciplines league, for nothing at all to pass your lips between sunrise and sunset is hardcore. Comparing that to giving up sugar, or chocolate, is like comparing a gentle park run to an Ironman challenge.

So respect to Christians4Ramadan, but respect to Muslims4Lent, too. Giving up something you like is never easy, even if you know the good times are coming back. After Lent comes Easter. After Ramadan comes Eid.

But there's more to it than that, of course, as anyone who has followed the excruciating saga of Larycia Hawkins and Wheaton College will realise. She was the professor who said she was going to wear a hijab in solidarity with Muslims during Advent and that, after all, we worship the same God. The firestorm that followed resulted in her losing her job.

In the evangelical world, expressions of solidarity and mutual good will across the Christian-Muslim divide are transgressive. They seem to imply that there's no difference between us, that every way to God is equally valid. They raise the spectre of syncretism, in which odd bits and pieces of two different religions are cobbled together to form something that isn't really either. If a Muslim can observe Lent or a Christian Ramadan, isn't the fundamental distinction between them being eroded?

Full disclosure: I think the whole Muslims4Lent thing is admirable, though I'm unlikely to reciprocate in Ramadan as I am just not spiritually tough enough. At its best, this is not about one religion trying to colonise another, or denying its own uniqueness; it's a statement of our shared humanity and an acknowledgment that we are children of one God.

That should not be a controversial thing to say: Malachi 2:10 says, "Do we not all have one Father? Did not one God create us?" and in Ephesians 3:14 Paul says, "For this reason I kneel before the Father, from whom every family in heaven and on earth derives its name."

Interestingly, this sort of practice, which is far more about mutual respect and an acknowledgment of a common tradition than it is about blurring the boundaries between faiths, used to be far more common, particularly where Christians and Muslims were used to living alongside each other. It's the transformation of religion into politics and ideology – begun by Islamists but enthusiastically embraced by Western culture warriors – that has made it rare.

In his wonderful 1997 book From the Holy Mountain, the record of a journey through the lands of Eastern Christianity made in the footsteps of the 6th-century monk John Moschos, William Dalrymple records other examples. He visits the shrine of a Muslim Sufi saint in Syria, for instance, and talks to the sheikh who looks after it. Many Christians went there for healing. The sheikh tells him of a Christian girl who spent the night at the tomb and was healed. "There is no difference between the Christians and ourselves on this matter," he says, "except that sometimes the Christians make the sign of the cross over the forehead of the person whom they want Nebi Uri to cure."

In Lebanon he meets a priest who tells him that Muslim Druze come to his church. "When they need babies or are ill or in difficulties they come here. They give oil and incense and are healed."

He tells Dalrymple: "In this part of the the world, for all our difficulties, religion has not just torn people apart. It has often brought them together. It is important to remember that."

Even the characteristic Muslim posture of prayer, kneeling with the forehead to the ground, was borrowed from Eastern Christians.

More full disclosure: I'm a Christian. I think syncretism is stupid and patronising. So when I say, "I believe in God", I'm saying I believe in the God who is revealed in Jesus Christ and incarnate in him alone. 

Does that mean that we can't respect and honour others' beliefs? For the life of me I can't see why. Of course there are limits on inter-faith observance. I wouldn't, for instance, think it appropriate for a Muslim to preach in a Christian church, or for a Christian to lead Muslim prayers. I would never do or say anything that implied Jesus was not Lord, and I wouldn't expect a Muslim to pray 'in Jesus' name' as an expression of solidarity. Our faiths are different. No one gains by pretending they're the same.

But no one gains, either, by pretending they're so different there's no common ground, or that we need to be resolutely hostile toward each other. And if we are defining ourselves in opposition to each other by our Christian and Muslim faiths, there's something very right about using the practices of those faiths as bridges of understanding and commitment to each other.

Muslims4Lent? Absolutely. Christians4Ramadan? I think I need to toughen up.

Follow Mark Woods on Twitter: @RevMarkWoods