The text:
The first explicit evidence ... is indeed sixth century, ...
is a shortened form. It seems intended to mean something like:
The earliest explicit evidence ... indeed dates from the sixth century, ...
or perhaps
The earliest explicit evidence ... indeed shows Chrisatian symbolish to have been present in the sixth century, ...
or more simply:
The earliest explicit evidence ... indeed is from the sixth century, ...
Saying "the evidence is sixth century" is, in my view, careless and therefore poor writing. However, I think most fluent speakers would correctly understand it to mean that the evidence is from that time, or shows Christian symbols to have been present in that time.
The problem is not the placement or use of the word "evidence", that is fine. The problem is the use of the verb form "is" without qualification, which seems to make an equation between the evidence and the century. Using instead a verbal phrase such as "is from", "dates from" or "belongs to" clarifies the meaning of the text.