secular: [13] Latin saeculum, a word of uncertain origin, meant ‘generation, age’. It was used in early Christian texts for the ‘temporal world’ (as opposed to the ‘spiritual world’), and that was the sense in which its derived adjective saeculāris passed via Old French seculer into English. The more familiar modern English meaning ‘non-religious’ emerged in the 16th century.
secular (adj.)
c. 1300, "living in the world, not belonging to a religious order," also "belonging to the state," from Old French seculer (Modern French séculier), from Late Latin saecularis "worldly, secular, pertaining to a generation or age," from Latin saecularis "of an age, occurring once in an age," from saeculum "age, span of time, generation."
According to Watkins, this is probably from PIE *sai-tlo-, with instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. Another theory connects it with words for "seed," from PIE root *se- "to sow" (see sow (v.), and compare Gothic mana-seþs "mankind, world," literally "seed of men").
Used in ecclesiastical writing like Greek aion "of this world" (see cosmos). It is source of French siècle. Ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years). In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s.