Well, OTL, you had that essentially after the reign of King Marian III with the Iberian kingdom in the 4th cent.until the Arab invasions. Then under the reigns of Tamar and David during the Georgian "golden age" of the 12th and early 13th Cent. until the Mongol invasions.
Not likely you'll get a Christianized kingdom much earlier than the 4th Century.
It had low penetration of Christian acceptance in a pagan culture at that time and was a top-down conversion process (King converts, subjects after), unlike Edessa in the late 2nd Cent. which was heavily Christianized before it became the state religion.