Does the concept 'spiritual immunity' have any meaning? I would hope so. But it needs interpreted. Would one want spiritual immunity if it means we are immune to spiritual things or spirituality itself? Perhaps not even Richard Dawkins would sign up for that. (He has described himself as a cultural Christian.)
Rather, we could consider spiritual immunity as defences we can access to help us resist negative influences on the quality of our spiritual lives.
Ephesians 6: 10-18 advises putting on " the whole armour of God". Just as our bodily immune system doesn't fend off every attack, it seems fair to assume the spiritual defence system will also be penetrated at times. But at least it is on offer!
But how does it work in practice? That could take a sermon or two to unpack!