There was a time when the emperor was thought of as a living god.
There was a time when the emperor was thought of as a god on earth.
There was a time when the emperor was regarded as/seen as a living god.
「かつて〜(の)時代がありました」は英語で「There was a time when 〜」です。
「天皇」は英語で、「emperor」です。
ちなみに、「皇后」は英語で「empress」と言います。
「〜と考えられていた」は「was thought of as ~」です。
「Was regarded as 〜」と「was seen as 〜」も同じ意味ですので、好きなのを使ったらいいと思います。
「現人神」は「living god(生きている神)」または「god on earth(地上にいる神)」がいいと思います。
There was a time when the emperor was thought of as a living god.
There was a time when the emperor was thought of as a god on earth.
かつて天皇は現人神だと考えられていた時代がありました。
There was a time when the emperor was regarded as a living god.
There was a time when the emperor was seen as a living god.
There was a time when the emperor was regarded as a god on earth.
There was a time when the emperor was seen as a god on earth.