From the Aramaic te'oma תאומא (twin) evolved the Hebrew tə'ōm אוםת and then the Bibical Greek Θωμᾶς (Thomas).

This was the name borne by one of the disciples of Christ who is best known for his scepticism about Christ’s resurrection (John 20: 24-9), hence “Doubting Thomas”. 

Before the Norman Conquest of 1066 the name is found only as the name of a priest or man of the cloth.  After this date the name and its variants gained in popularity.

The demands of bureaucracy formally introduced by feudal lords in the 11th century, to define the boundaries and families within their fiefdoms, crystallised the need for personal identification and accountability, and surnames became in general use from this time onwards.    

Also, because of the French influence, the pronunciation changed from /θ/  to /t/ and, consequently, the spelling also.  e.g. Thomlin to Tomlin.

One of the earliest recordings of the Tomlin name in the U.K. is believed to be in the Subsidy Rolls of A.D. 1327.  This subsidy was granted by the First Parliament of King Edward III (of England) to meet the expenses of the war with Scotland. The following were named  :-    Robert Thomelyn (of Cambridgeshire),  John Thomelyn (of Sussex),  John Thomelyn (of Somerset)  & John Tomelyn (also of Somerset).

One of the earliest recordings of the Tomlin name in the New World is believed to be when Matthew Tomlin arrived in Virginia in 1636.