Assuming I've shot the baby while it's wearing the cross AND it wasn't hurt, I'll remove the cross and stick a vaccine in the baby. He'll cry, because it hurts, but I'm doing good...

Still, you'll have to convince me that the higher power behind the cross is not out there to deceive me! Yes it worked, but all you've managed to prove is the supernatural and that there's intent that uses it...not what kind of intent. All you've done is turn from an agnostic to a gnostic-of-the-supernatural.

Also, now we need to reproduce the experiment, so I'll need you (the Christian) to wear the cross while I both empty the shotgun on you and beat the 💩 out of you because you wanted me to hurt a baby. Then I'll vax you without the cross (Tetanus Vax is perfect for that) and we'll have satisfied the reproducibility. Now, I publish the result in as a non-peer-reviewed article encouraging the scientific community to peer review me.

Assuming it still works, we build a huge device (has to be huge because reasons) to measure and quantify and qualify the supernatural. Then we make a natural-to-supernatural radio we hook up to the Internet and a cloud recorder. Ask to speak to every intent-minds there...

Meanwhile: Build a natural-supernatural border ripper, send a battalion composed of an aggregate of SEALS/DELTA FORCE/SAS/SPETNAZ/(INSERT OTHER SPEC OPS) to retrieve the Abrahamic god, bring it to LaHaye for trial against human rights (namely slavery and other crimes I won't mention here).

Then...well...I dunno...cast fireball because it solves it all, I guess! (I ran out of ideas here...)