Arabic numerals
The first mentions of the numerals in the West are found in the "Codex Vigilanus" of 976.
Isopsephy
Until Arabic numerals were adopted and adapted from Indian numerals in the 8th and 9th centuries AD, and promoted in Europe by Fibonacci of Pisa with his 1202 book "Liber Abaci", numerals were predominantly alphabetical. For instance in Ancient Greece, Greek numerals used the alphabet. It is just a short step from using letters of the alphabet in everyday arithmetic and mathematics to seeing numbers in words, and to writing with an awareness of the numerical dimension of words.