#1. 'The Celtic cross is a form of Christian cross featuring a nimbus or ring that emerged in Ireland and Britain in the Early Middle Ages. A type of ringed cross, it became widespread through its use in the stone high crosses erected across the islands, especially in regions evangelized by Irish missionaries, from the 9th through the 12th centuries.' Taken from Wikipedia.
#2. The number 8 in Christianity represents rebirth, a new beginning, when man is ultimately resurrected from death into eternal life. A fascination Zodiac appeared to harbor with his desire for the afterlife.
#4. It has been speculated that the three eights may possibly be three Taurus symbols, in view of what vaguely looks like an upside down Aries symbol to their right - but this may be misleading, in view of the general consensus they are in fact three eights.
#5. The crude looking symbol may actually be an anchor. 'Christians adopted the anchor as a symbol of hope in future existence because the anchor was regarded in ancient times as a symbol of safety. For Christians, Christ is the unfailing hope of all who believe in him.' Taken from Wikipedia. Again, referencing a future existence or afterlife.
#7. The number 888 represents Jesus or Christ the Redeemer in Christian numerology. See here.
#8 And finally number eight, which has been covered before. The Zodiac was the ultimate egotist, but did he sign off the 340 Cipher by introducing Jesus Christ once more, generated directly from his own pseudonym, before transferring it from the 340 Cipher to 13 Symbol Cipher. It is quite remarkable that the 340 Cipher has withstood almost 50 years of the best modern technology we can throw at it, unless it is simply undecipherable. Here we shall focus on just eight characters, all present on the last line, as a form of signature. These eight characters at the foot of the 340 Cipher are in essence all transferred to his next cipher, as though he was linking the two ciphers together.
ZOD produces three zero's, the IAC produces 888. These two incorporated together gives us the three circled 8's found on his next cipher - as though he was carrying them forward. It is also apparent on the 13 Symbol Cipher, that either side of the three circled 8's are the Zodiac crosshair and 'anchor' symbol, both of which can be seen on the final line of the 340 Cipher effectively in the same position. So the author, in essence, carried eight characters from one cipher to another. Finally, the 408 Cipher was separated into three communications of eight lines, with the letters K and M featuring exclusively at the end of each communication. The exact letters encircled by the three eights on the 13 Symbol Cipher.
All of the above is subjective and does not necessarily indicate the Zodiac Killer was inspired by a warped religious calling. It is simply a matter of faith.