how did he adopt the initials of christ?
The Chi-Rho symbol was used by the Roman Emperor Constantine I (r. 306–337) as part of a military standard (vexillum). Constantine's standard was known as the Labarum.
Chi Rho
The Chi-Rho symbol was used by the Roman Emperor Constantine I (r. 306–337) as part of a military standard (vexillum). Constantine's standard was known as the Labarum. Early symbols 
he was roman emperor from 306-337 ce becoming the first emperor to convert to christianity Constantine does he have any other name? Constantine the Great what is his birth name? Flavius Valerius Constantinus what were his religious policy? He was the first emperor to stop the persecution of Christians and to legalize Christianity, along with all other religions/cults in the Roman Empire
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Chi Rho [SEP] Introduction
The Chi Rho (; also known as "chrismon") is one of the earliest forms of christogram, formed by superimposing the first two (capital) letters—chi and rho (ΧΡ)—of the Greek word (Christos) in such a way that the vertical stroke of the rho intersects the center of the chi. The Chi-Rho symbol was used by the Roman Emperor Constantine I (r. 306–337) as part of a military standard (vexillum). Constantine's standard was known as the Labarum. Early symbols similar to the Chi Rho were the Staurogram () and the IX monogram (). In pre-Christian times, the Chi-Rho symbol was also to mark a particularly valuable or relevant passage in the margin of a page, abbreviating "chrēston" (good).