You'd have to ask Edmund Spenser about that; the word are his (Cherubins as well!)
Or you could ask the OED, as I have just done:
"...The Latin form seraphin , which is found in many MSS. of the Vulgate, and is the source of all the forms used in English down to the 16th cent. (as well as of those in the Romanic languages), coincides with the Aramaic serāphīn , but it is very doubtful whether it is more than a scribal error or a euphonic alteration. Compare French séraphin (serafin , 12th cent.), Provençal serafi , Spanish serafin , Portuguese seraphim , Italian serafino (all masculine singular).
In the Latin liturgical passages from which the word first became widely known, it was probably originally apprehended correctly as a plural, and readers of the Latin Bible would be guided aright by the syntax of Isa. vi. 2; but there is evidence that ‘Cherubim and Seraphim’ were often supposed to be the names of two individual angels. From the 15th to the 18th cent. the English plural ending was often appended, but seraphin as a singular = ‘one of the seraphim’ does not appear in English till late in the 16th cent. (the form seraphim in this use not till the 17th cent.). After the introduction (perhaps by Milton) of the form seraph , the misuse of the plural forms in singular sense gradually became rare, and it is now obsolete."
Or you could ask the OED, as I have just done:
"...The Latin form seraphin , which is found in many MSS. of the Vulgate, and is the source of all the forms used in English down to the 16th cent. (as well as of those in the Romanic languages), coincides with the Aramaic serāphīn , but it is very doubtful whether it is more than a scribal error or a euphonic alteration. Compare French séraphin (serafin , 12th cent.), Provençal serafi , Spanish serafin , Portuguese seraphim , Italian serafino (all masculine singular).
In the Latin liturgical passages from which the word first became widely known, it was probably originally apprehended correctly as a plural, and readers of the Latin Bible would be guided aright by the syntax of Isa. vi. 2; but there is evidence that ‘Cherubim and Seraphim’ were often supposed to be the names of two individual angels. From the 15th to the 18th cent. the English plural ending was often appended, but seraphin as a singular = ‘one of the seraphim’ does not appear in English till late in the 16th cent. (the form seraphim in this use not till the 17th cent.). After the introduction (perhaps by Milton) of the form seraph , the misuse of the plural forms in singular sense gradually became rare, and it is now obsolete."
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