You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Mosaic

Context:
In Rome, Nero and his architects used mosaics to cover some surfaces of walls and ceilings in the Domus Aurea, built 64 AD, and wall mosaics are also found at Pompeii and neighbouring sites. However it seems that it was not until the Christian era that figural wall mosaics became a major form of artistic expression. The Roman church of Santa Costanza, which served as a mausoleum for one or more of the Imperial family, has both religious mosaic and decorative secular ceiling mosaics on a round vault, which probably represent the style of contemporary palace decoration.

Question:
When did figure wall mosaics become the highest form of artistic expression?

the Christian era