It doesn't seem to have any of the "to become" meaning that the normal meaning has. In most translations there is usually no mention of "becoming" at all. Then what purpose does naru serve then?
Also 大いなる. 大いなる力 referred to an already existing great power in a show and it didn't have to 'become' great.
My answer or quote is almost same with that of Aketoshi san, so I don't understand why his answer was downboted.
Anyhow, from the link,
It comes from the old Japanese way of use.
It is the remnant of the 4th form of the 古典日本語( Old Japanese ), which in much older time, being にあり、but shortened to なり。
The translation of the 4th form.
4 状態・性質を表す。 三寸ばかりなる人、いとうつくしうてゐたり。（竹取物語）
4 denote the condition, nature, characteristic.
There was a charming boy who is about 三寸 tall. ( Sanzun ( about 9.18cm )) ( 竹取物語 ( Taketori Monogatari ) )
The auxiliary's conjucated form is, as you can see too,
So 大いなる、完全なる、聖なる are divided to
大い(adjective, meaning big)+なる(this auxiliary)
完全(noun, meaning perfect)+なる(same)
聖(noun, meaning holy)+なる(same)
respectively, taking the attributive form of the conjucation.
So, Aketoshi san's answer is not wrong.
Since this form is the inheritance of the old meaning, today's "to become" can not be applied.
It's an old form of な, essentially. More specifically, it's the attributive of なり, an old verb equivalent to だ・である. なる is a little bit broader than な in use, however; you can place なる after basically any noun. A common example is 母なる大地 - Mother Earth. Sometimes it's necessary even in modern language, but most of its modern use is simply to make things sound more grandiose.
