Priests used to wear their ring on the fore-finger (which represents the Holy Ghost) in token of their spiritual office. (See Wedding Finger.)
The ring finger represents the humanity of Christ, and is used in matrimony, which has only to do with humanity. (See Finger Benediction.)
The fact has nothing to do with the question; that the ancients believed it is all we require to know. In the Roman Catholic Church, the thumb and first two fingers represent the Trinity: thus the bridegroom says, “In the name of the Father,” and touches the thumb; “in the name of the Son,” and touches the first finger; and “in the name of the Holy Ghost” he touches the long or second finger. The next finger is the husband's, to whom the woman owes allegiance next to God. The left hand is chosen to show that the woman is to be subject to the man. In the Hereford, York, and Salisbury missals, the ring is directed to be put first on the thumb, then on the first finger, then on the long finger, and lastly on the ring-finger, quia in illo digito est quadam vena procedens usque ad cor.
The ring finger. Mr. Henry Swinburne, in his Treatise of Spousals, printed 1680 (p. 208), says: “The finger on which this ring [the wedding-ring] is to be worn is the fourth finger of the left hand, next unto the little finger; because by the received opinion of the learned ... in ripping up and anatomising men's bodies, there is a vein of blood, called vena amoris, which passeth from that finger to the heart.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894