Theologian's important date with university filling in text (7)
I believe the answer is:
erasmus
'theologian's' is the definition.
(I've seen this before)
'important date with university filling in text' is the wordplay.
'important date' becomes 'era' (I've seen this before).
'with' says to put letters next to each other.
'university' becomes 'U' (abbreviation for university).
'filling in' is an insertion indicator.
'text' becomes 'SMS' (abbreviation for Short Message Service).
'u' going inside 'sms' is 'smus'.
'era'+'smus'='ERASMUS'
(Other definitions for erasmus that I've seen before include "Desiderius -, Dutch scholar, humanist" , "Dutch philosopher" , "Learned person" , "Was this Dutch Renaissance man a strange masseur?" , "Humanist critical of Church" .)