I have a page of notes in front of me: preparation for a talk from the Psalms and the Gospel of John. The page is full of tiny writing, and - in keeping with the shorthand custom I learned while at seminary - has no small amount of X's and Θ's.
The Greek word for God is ΘΕΟΣ (Theos), and so I write the first letter, a Theta (Θ) as a shorthand for God.
Similarly, the Greek word for Christ is ΧΡΙΣΤΟΣ (Christos), and so I write the first letter, a Chi (Χ) as a shorthand for Christ.
The Early Christians did the same thing. The reason that the Fish became an emblem for early Christianity was not because of the large number of fisherman among the early disciples. The reason early Christians identified with a fish was because it had credal value. The Greek word for fish, ΙΧθΥΣ (Ichthus, from which we get ichthyology: the study of fish), also became an acronym for the foundational truths of the faith. The early Christians were the ones who believed in Jesus Christ, the Son of God (and) Savior).
In Greek, you would write those words this way:
θΕΟΥ (of God)
Put the first letters together... and it spells "fish".
So, this is just to say that I'm one of those Christians who has a list in my house that say "Xmas presents". And I mean nothing but honor in writing it this way.
Just in case anyone was wondering.