Papyrus is expensive. Scripture is repetitive. The earliest Christian texts used a clever set of abbreviations to save space and time.
The basic form of a nomen sacrum is to take the first letter of the word and the last letter of the word, and put a bar over the top to indicate the abbreviation. The Greek word for Lord, for example, is Κύριος. The abbreviation is ΚΣ (or, ΚΥ if it’s possessive).
(Side note 1: this is not the same as the chi rho, the so-called Christogram today seen mostly in the X in X-mas. It’s pretty similar, though.)
Reading ancient Christian scriptures, you needed to know a good number of these sacred abbreviations to make any sense of the text. And even the earliest New Testament scriptures use them. Interestingly enough, it wasn’t just a matter of convenience: the words abbreviated in this way were all religiously significant: God, spirit, saviour, Jerusalem… Words that came up a lot but that weren’t sacred (like “and”) didn’t get abbreviated.
(Side note 2: many other non-sacred abbreviations have been used by scribes, and we see the remnants of them today in forms like RIP and &, but that’s a larger topic.)
Knowing the nomina sacra can also help when viewing early Christian art. The picture above, for example, identifies Jesus Christ with only four letters.