Why did a kingdom in Ethiopia mint its own gold coins?

In the 3rd century the prophet Mani listed the four great powers of his world: Persia, Rome, China and Aksum. Aksum was a kingdom in the highlands of what is now Eritrea and northern Ethiopia, and it earned its place by sitting on the sea road between Rome and India.

Its harbour was Adulis, on the Red Sea. From about the first century, ships sailed from Egypt down the Red Sea and used the monsoon winds to cross the Arabian Sea straight to southern India. Adulis became the main port for exporting African ivory, incense, gold and other goods. A trading guide, the Periplus of the Erythraean Sea, mentions Axum already in the mid-first century.

What set Aksum apart was how it announced itself. It was one of the first African polities to issue its own coins, struck in gold, silver and copper and stamped in Greek and in its own language, Geʽez. In the ancient world, minting coins was an act of great importance: it proclaimed that Aksum considered itself the equal of its neighbours. Aksumite coins from the reign of Endybis have been dug up as far away as Caesarea and southern India.

In the 4th century King Ezana made Christianity the state religion, and his coins swapped older symbols for the cross. At home Aksum raised giant obelisks over tombs, the most famous of which is still called the Obelisk of Aksum. Then, from the 7th century, minting stopped and a slow decline began.