Mutilated coins with possible Christian significance:
(click for images)

1. Æ semis of Domitian. The cross of punched circles possibly represents the five wounds of Christ.
   
   
2. 4th C. Æ 3 cut to form a cross.
   
   
3. Æ 17, the obverse smoothed and engraved with a star pattern, possibly representing a Chi-Rho. The reverse type is eagle stg. l. with wings closed, probably Ptolemaic. This would mean the coin was already very old when altered. For a very similar piece, see CNG XXXIII, March 15, 1995 Sale, lot 1883, which is an altered Æ of Rheskouporis V (318-337 A.D.), Bosporus, and thus during Christian times. CNG mentions however these may have been used as gaming tokens. For a larger example (Antioch, Pisidia) with two intersecting lines instead of three, plus an interesting discussion, see B. E. Levy, "Another Converted Roman Coin?" Gazette Numismatique Suisse, vol. 32, pp. 40-42, 1982. Click here for another example: obv: rev: