Most ancient conversions are known only from what later writers said about them. Ezana's is different, because he left two independent series of contemporary evidence and they agree. The first is the coinage. Aksum was one of the few states of its era to mint its own gold, and Ezana's earlier issues carry the disc and crescent that stood for the older gods of the kingdom. Partway through the reign the symbol changes to a cross, and the kingdom becomes one of the first in the world to put the Christian sign on its currency, ahead of the Roman mints in doing so consistently.
The second is his own royal inscriptions, cut on stone stelae at Aksum in Ge'ez, Sabaean and Greek. The earlier texts open by naming Mahrem, the war god the Greek versions render as Ares, and describe the king as his son. A later inscription, recording a campaign against the Noba on the middle Nile, invokes instead the Lord of Heaven, who is in heaven and on earth, in language that avoids naming Christ directly. A later text still is explicitly Trinitarian. The sequence lets historians watch a royal religion change over a single reign rather than infer it.
What the evidence does not settle is how deep the change ran outside the court in Ezana's own lifetime. The monumental stelae field at Aksum, the great carved granite markers of the pre-Christian royal burials, appears to stop with his period, which is usually read as the point at which the old funerary religion lost its royal patron. Church building at Aksum on any scale comes later, and the christianization of the countryside is generally credited to the monastic movement of the Nine Saints in the following century.