The earliest known church inscription (found near Damascus) is Marcionite, and dates to 318 CE.
Συναγωγη Μαρκιωνιστων κωμ(ης)
Λεβαβων του κ(υριο)υ και σω(τη)ρ(ος) Ιη(σου) Χρηστου
προνοια(ι) Παυλου πρεσβ(υτερου) — του λχ’ ετους.
[“The meeting-house of the Marcionists, in the village of
Lebaba, of the Lord and Savior Jesus The Good.
Erected by the forethought of Paul the elder — In the year 630.”]
χριστου (Christoy – pronounced close to “crystal”) means “the anointed”. χρηστου (Chrestoy – pronounced “chreestou”) means “the good” or “the useful”.
The name Christian, however, so far as its meaning goes, bears the sense of anointing. Even when by a faulty pronunciation you call us Chrestians (for you are not certain about even the sound of this noted name), you in fact lisp out the sense of pleasantness and goodness.
– Tertullian in Ad Nationes 1 c. 200 CE
Though “Chrestians” might be a designator for “the useful ones”, which was a common name for slaves. And then there’s this:
Consequently, to get rid of the report, Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Chrestians by the populace. Chrestus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus, and a most mischievous superstition, thus checked for the moment, again broke out not only in Judaea, the first source of the evil, but even in Rome, where all things hideous and shameful from every part of the world find their centre and become popular. Accordingly, an arrest was first made of all who pleaded guilty; then, upon their information, an immense multitude was convicted, not so much of the crime of firing the city, as of hatred against mankind. Mockery of every sort was added to their deaths. Covered with the skins of beasts, they were torn by dogs and perished, or were nailed to crosses, or were doomed to the flames and burnt, to serve as a nightly illumination, when daylight had expired.
– Tacitus, Annals 15.44 c.117 CE
Interesting confusion over “Christian” (χριστιανοι – the anointed ones) and “Chrestian” (χρηστιανοι – the good ones). What a difference one iota makes!