I got a question for you: During the first two centuries of Christianity, the cross was rare in Christian iconography, as it depicts a purposely painful and gruesome method of public execution and Christians were reluctant to use it. A symbol similar to the cross, the staurogram, was used to abbreviate the Greek word for cross in very early New Testament manuscripts such as P66, P45 and P75, almost like a nomen sacrum (nomina sacra). The extensive adoption of the cross as Christian iconographic symbol arose from the 4th century.

Answer this question: when did the cross become a symbol of christianity?
Let's think first:
This is the relevant information: The extensive adoption of the cross as Christian iconographic symbol arose from the 4th century.
Therefore, the answer is the 4th century.