'Secular', as mplungjan or another good dictionary will tell you, comes from the Latin saecula. There is no perfect translation for this, but 'age' , as in 'spirit of the age' is fairly close.
And that is the cause of the opposite meanings. To an economist or a journalist who is used to thinking of a decade as long enough for entire theories to rise and fall, an age is the longest period imaginable; durable ideas can be age-old and this age is synonymous with 'this world' or 'everything'. But to a theologian, who naturally contrasts 'time' with 'eternity' (hence temporal as both 'time-related' and 'not related to the church') an age is a fleeting moment. Isaac Watts wrote "A thousand ages in Thy sight/ Are as an evening gone"; the mediaeval Dies Irae has the line "Solvit saeclum in favilla", 'he dissolves the age into ashes'. Nineteenth century freethinkers, looking for a term to reflect a philosophy with no belief in eternity, reasonably chose secularism.