Hastings' Dictionary of the New Testament
Abba is the emphatic form of the Aram. word for ‘father’ (see Dalman, Aram. Gram. p. 98, for אב and its various forms; also Maclean, in Dict. of Christ and the Gospels , s.v.). It is found only in three passages in the NT, viz. Mark 14:36, Romans 8:15, Galatians 4:6; in each case ὁ πατήρ is subjoined to Ἀββᾶ, the whole expression being a title of address. [The use of ὁ πατήρ, nominative with the article, as a vocative, is not a Hebraism, as Lightfoot thought, but an emphatic vocative not unknown to classical Greek and common in the NT: ‘nearly sixty examples of it are found in NT’; sea Moulton, Gram. of NT Greek, Edinburgh, 1906, p. 70.]
Lightfoot on Galatians 4:6 argues that the bilingual expression is a liturgical formula originating with Hellenistic Jews, who, while clinging to the original word which was consecrated by long usage, added to it the Greek equivalent; but he supports an alternative theory that it took its rise among Jews of Palestine after they had become acquainted with the Greek language, and is simply an expression of importunate entreaty, and an example of that verbal usage whereby the same idea is conveyed in different forms for the sake of emphasis. As illustrations of this repetition, he quotes Revelation 9:11 (Ἀπολλύων, Ἀβαδδών) Revelation 12:9; Revelation 20:2 (Σατανᾶς, Διἀβολος). Thayer, in Hasting's Dictionary of the Bible (5 vols) (s.v.), points out that, though devotional intensity belongs to repetition of the same term (e.g. κύριε, κύριε), it is also expressed by such phrases as ναὶ ἀμήν, ‘Hallelujah, Praise the Lord,’ where the terms are different. The context of each passage where ‘Abba, Father’ is found appears to prove that the Greek addition is not merely the explanation of the Aramaic word, such as, e.g., St. Peter might have added in his preaching-a custom to be perpetuated by the Evangelists, as suggested by the passage in Mk.; but is rather an original formula, the genesis of which is to be sought further back, perhaps in the actual words used by our Lord Himself. Thus Sanday-Headlam on Romans 8:15 (International Critical Commentary , 1902) remark:
‘It seems better to suppose that our Lord Himself, using familiarly both languages, and concentrating into this word of all word such a depth of meaning, found Himself Impelled spontaneously to repeat the word, and that some among His disciples caught and transmitted the same habit. It is significant however of the limited extent of strictly Jewish Christianity that we find no other original examples of the use than these three.’
Thus, the double form is due to the fact that the early Christians were a bilingual people; and the duplication, while conveying intensity to the expression, ‘would only be natural where the speaker was using in both cases his familiar tongue.’ F. H. Chase (Texts and Studies i. iii. 23) suggests that the phrase is due to the shorter or Lucan form of the Lord’s Prayer, and that the early Christians repeated the first word in the intensity of their devotion, coupling a Hellenistic rendering with the Aramaic Abba. He argues that the absence of such a phrase as ὅ ἐστιν, or ὅ ἐστι μεθερμηνευόμενον, in Mark 14:36 is due to the familiarity of the formula; and that, while the Pauline passages do not recall Gethsemane, they suggest the Lord’s Prayer as current in the shorter form. Moulton (op. cit. p. 10), combating Zahn’s theory that Aramaic was the language of St. Paul’s prayers-a theory based on the Apostle’s ‘Abba, Father’-remarks that ‘the peculiar sacredness of association belonging to the first word of the Lord’s Prayer in its original tongue supplies a far more probable account of its liturgical use among Gentile Christians.’ He mentions the analogy (see footnote, loc. cit.) of the Roman Catholic ‘saying Paternoster,’ but adds that ‘Paul will not allow even one word of prayer in a foreign tongue without adding an instant translation’; and further refers to the Welsh use of Pader as a name for the Lord’s Prayer.
It seems probable (1) that the phrase, ‘Abba, Father,’ is a liturgical formula; (2) that the duality of the form is not due to a Hebraistic repetition for the sake of emphasis, but to the fact that the early Christians, even of non-Jewish descent, were familiar with both Aramaic and Greek; (3) that Abba, being the first word of the Lord’s Prayer, was held in special veneration, and was quoted with the Greek equivalent attached to it, as a familiar devotional phrase (like Maran atha [1 Corinthians 16:22], which would be quite intelligible to Christiana of Gentile origin, though its Greek translation, ὁ Κύριος ἐγγός [Philippians 4:5], was also used; cf. Did. 10. 5, where ‘Maran atha’ and ‘Amen’ close a public prayer); and (4.) that our Lord Himself, though this cannot be said to be established beyond doubt, used the double form in pronouncing the sacred Name, which was invoked in His prayer.
In conclusion, it should be noted that, while the phrase is associated with the specially solemn occasion of the Gethsemane agony, where our Lord is reported by St. Mark to have used it, both examples of its use in the Pauline writings convey a similar impression of solemnity as connected with the Christian believer’s assurance of sonship-and sonship (let it be noted) not in the general sense in which all humanity may be described as children of God, but in the intimate and spiritual connotation belonging to υἱοθεσίαν, or ‘adoption,’ into the family of God.
Literature.-See article ‘Abba’ in Hasting's Dictionary of the Bible (5 vols) , Dict. of Christ and the Gospels , and Jewish Encyclopedia , an art in Expository Times xx.  358, and the authorities cited above.
R. Martin Pope.
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Hastings, James. Entry for 'Abba'. Hastings' Dictionary of the New Testament. https://www.studylight.org/dictionaries/eng/hdn/a/abba.html. 1906-1918.