Why do many professing Christians use the secular philisophical use of the word "Foreknowledge"?

All human language is secular in nature. The bible only borrows language in order to communicate spiritual truth. The bible never igns new meaning to existing words but rather uses the currently existing word in that culture which best communicates the idea that it wishes to get across. So the best way to understand the meaning of a word used in the bible is to see how that word was used in the contemporary society and culture in which the bible was written - and that includes examining other biblical usages of the word in that time period as well. But there is no such thing as a strictly biblical word. For words are simply the means of communicating divine truth in human terminology.