Why do we need to? If we are true Christians we all have a common bond and understand each others beliefs. We share a familial bond because each of us has the same Spirit within. We do not need to prove anything to each other.
For unbelievers to want proof is pretty par for the course, but whatever we tell them makes not difference they will still be unbelievers so why waste time.
If you want proof then ask God directly.
Witness. It’s not a waste of time.
“Now these were more noble than those in Thessalonica, in that they received the word with all readiness of mind, examining the Scriptures daily to see whether these things were so.”
(Acts 17:11, WEB)