Actually proving that you own copyright can be quite difficult. But the more evidence you have of the steps you took in creating the artwork, the better. You mention that 'we' took part in this process. It would be sensible to get short but detailed, written and dated statements from each of the participants stating what they did or saw others do during the creation process. Get the individuals to include dates of when they witnessed the various stages and who did what. Ideally the statements should be in the person's own words, unprompted by anyone else. A statement of truth
should be added at the end. Keep these statements safely along with the rest of the files etc.
Ideally you need to carefully store away the original artwork as best evidence of your creativity. The weak point in the evidential chain is the high resolution photographs, since all they show is that you copied some calligraphy, not that it was necessarily your own copyright work. For the subsequent steps of refining the characters with software, the digital files should be OK. Although the files will be dated, it is not that complicated to fake the dates on digital files, so you could think about getting the files notarised. This means getting a notary public to certify that the files (on some physical medium like a CD, DVD or memory stick) existed and were in your possession at a specific point in time. However you don't need to use a notary (which would be an extra cost). Just get a friend or someone senior in your church who was unconnected with the calligraphy project to certify that on a certain date they saw the documents and witnessed them being sealed in an envelope, over the seal of which they affixed their signature, which in turn should be sealed with sellotape. This doesn't prove that you own the copyright, but it does establish a date from which the claim can be reasonably verified. The envelope should be stored and not opened at any stage until this was required by the court or a suitable arbitrator. Remember that copyright lasts for 70 years after the death of the last of the joint authors, and a claim could, theoretically, arise long after any actual witnesses had died.
Thus if anyone did copy your calligraphy and claim they owned the copyright they would need to establish an earlier existence of their artwork.
Of course there are two other aspects of the originality criterion which it is hard to provide evidence for. The first is whether your calligraphy contains sufficient differences from pre-existing or generic works of a similar nature, and the second is evidence that your first artwork wasn't directly copied from another source, and just tweaked using a computer. However since, apart from the witness statements mentioned above, neither of those issues can be adequately addressed unless and until a claim is made against you, there is nothing more to be done about that.
If you should ever be in the unfortunate position of going to court over the copyright, the evidence you have should reasonably establish your claim (assuming that you were the claimant in the case) and the onus would then be on your opponent to either disprove your evidence or provide better evidence of their own copyright claim.
As for court cases of a similar nature, take a look at Bodo Sperlein Ltd v Sabichi Ltd & Anor  EWHC 1242 (IPEC)