1. Many ancient number systems, including Greek and Hebrew, used the letters of their alphabets as numbers. (The Hebrew number system was similar to the above; both were decimal systems.) Numbers had names as well, but often (as in trade-ledgers and page-numbering, e.g.) a number was written as a sequence of letters (no more than one from the same tier) read from left to right (Greek) or right to left (Hebrew), usually with an overstroke or other marking. (Numbers higher than 999 used special markings to indicate the multiplicative power of ten involved.)
2. Because letters were used as numbers, every word and name had a numeric value, calculated by adding the values of the individual letters.
kosmos ('world') = 600 (20 + 70 + 200 + 40 + 70 + 200)
cwmas ('Thomas') = 1050 (9 + 800 + 40 + 1 + 200)
ihsous ('Jesus') = 888 (10 + 8 + 200 + 70 + 400 + 200)
IH, IS, IHS (nomina sacra) = 18, 210, 218, respectively
Having a number or numeric value is not to be confused with being a number. The nomen sacrum 'IH', for example, was also the number 18 (both being overstroked). The nomen sacrum 'IC', on the other hand, had the numeric value 210, although it wasn't a number in Greek, unless read from right to left as in Hebrew (the Greek number 210 being 'CI'). Most words and names weren't numbers read in either direction, but they all had a numeric value (albeit almost never unique).
3. It is important to realize that the use of this system was natural and normal for every educated Greek-speaker, even semi-educated tradespeople. Because we ourselves use special symbols (the numerals 0-9), the Greek and similar other early number systems seem strange to us. But they would not have been strange to the people who used them. (To put yourself in their mindset, imagine that when you look at 'IZ + NH', your brain automatically registers '17 + 58'.) Everyone who knew their ABG's ipso facto knew their 123's, and would thus have learned early on how to calculate the numeric value of words and names. It is therefore a mistake to assume that educated early Christians were unaware of the numeric values of words and names that were theologically important to them.
Michael Grondin, Jan 2016, rev 16 Sep 2017