According to the post-biblical Christian tradition, the apple is the fruit of the tree of knowledge of good and evil eaten by Adam and Eve in the Garden of Eden in defiance of God’s commandment.

However, in the Book of Genesis, the type of fruit eaten by Adam and Eve is not specified.

In the King James Version (1611), the two verses of the Book of Genesis (2:16-17) in which God’s commandment is given are:

16 And the Lord God commanded the man, saying, Of euery tree of the garden thou mayest freely eate.
17 But of the tree of the knowledge of good and euill, thou shalt not eate of it: for in the day that thou eatest thereof, thou shalt surely die.

In the Latin Vulgate, they are:

16 praecepitque ei dicens ex omni ligno paradisi comede
17 de ligno autem scientiae boni et mali ne comedas in quocumque enim die comederis ex eo morte morieris

And this Latin text is literally translated, in the Douay-Rheims Bible (1609), as:

(1635 edition)
16 And he commanded him saying: Of euerie tree of Paradise eate thou: 17 But of the tree of knowledge of good & euil, eate thou not. For in what day soeuer thou shalt eate of it, thou shalt dye the death.

The identification of the forbidden fruit as the apple appears to arise in the post-classical Latin tradition. (In the Talmud, it is variously identified as the grape, the fig, or wheat.)

This post-classical tradition used the Latin mālum, a word designating the apple, in the sense of any tree-fruit fleshy on the outside, and having a kernel within (as opposed to nux, nut, that is, any fruit with a hard shell or rind). Hence, mālum was also applied to the peach, the quince, the orange, the lemon, the pomegranate, etc.

To identify the forbidden fruit, the post-classical Latin tradition also used pōmum, a word designating fruits of any kind: apples, cherries, nuts, berries, figs, dates, etc.

Almost identically, English apple was originally a general term for all kinds of fruits other than berries, including even nuts. In fact, apple and berry are the only Anglo-Saxon fruit names, the rest being of Latin or ‘exotic’ origin. This is why apple was commonly used in describing foreign fruits, which explains for example the word pineapple, from the resemblance of the fruit to a pine cone.

In French, from Latin pōmum, pomme was used to identify the forbidden fruit. Just like Latin mālum and English apple, pomme originally designated a variety of fruits. For example, the mid-13^th century term poume de paradis designated the banana. The word pomme de pin means pine cone. And pome grenate, literally ‘apple having many seeds’, has given English pomegranate.

The choice of the apple to represent the forbidden fruit may have been influenced by the potential pun between Latin mālum, apple, and Latin mălum, meaning evil (neuter form of the adjective mălus, bad, evil, used as a noun). In the above-mentioned Latin verses from the Vulgate, măli is the genitive of mălum.

These two Latin words however are of distinct origins. In mālum, apple, the vowel a is long, but in mălum, evil, it is short. The former word is from Greek μῆλον (= melon), meaning apple, fruit, the latter from Greek μέλας (= melas), meaning black (cf. words such as melancholy, literally black bile).

The choice of the apple may also have been influenced by the Song of Solomon in the Old Testament, in which apples are an erotic symbol:

(King James Version)
2:3 As the apple tree among the trees of the wood, so is my beloued among the sonnes. I sate downe vnder his shadow with great delight, and his fruit was sweete to my taste.
2:4 Hee brought mee to the banquetting house, and his banner ouer mee, was loue.
2:5 Stay me with flagons, comfort me with apples, for I am sicke of loue.
[…]
7:7 This thy stature is like to a palme tree, and thy breasts to clusters of grapes.
7:8 I said, I will goe vp to the palme tree, I will take holde of the boughes thereof: now also thy breasts shalbe as clusters of the Uine, and the smell of thy nose, like apples.

There may also have been some association with apples as featured in Greek mythology. In particular, the apple of discord was the golden apple inscribed “For the fairest”, fabled to have been thrown by Eris, the personification of discord, into the assembly of the gods, and contended for by Hera, Athena, and Aphrodite.

The word apple was first used to designate the forbidden fruit in an Anglo-Saxon poem, Genesis, written about 1000. This poem is not a direct translation into Anglo-Saxon of the Old Testament Book of Genesis. Rather, it is an effort to retell the story in the poetry and style of the Germanic Epic, a style still popular with the Anglo-Saxons at the time. Eve, deceived, deceives Adam:

Sum heo hire on handum bær, sum hire æt heortan læg,
æppel unsælga, þone hire ær forbead
drihtna drihten, deaðbeames ofet.
translation:
One fruit she bore in her hands, another lay at her heart,
apple of disaster, which the Lord of Lords
had earlier forbidden, crop of the Death-Tree.