KJV Gal 2:11 But when Peter was come to Antioch, I withstood him to the face, because he was to be blamed. Gal 2:12 For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision. Gal 2:13 And the other Jews dissembled likewise with him; insomuch that Barnabas also was carried away with their dissimulation.
Peter appears quite fearless in his dealings with the unbelieving Jews despite the fact that they had thrown him in prison and had killed Jesus:
KJV Act 5:25 Then came one and told them, saying, Behold, the men whom ye put in prison are standing in the temple, and teaching the people. Act 5:26 Then went the captain with the officers, and brought them without violence: for they feared the people, lest they should have been stoned. Act 5:27 And when they had brought them, they set them before the council: and the high priest asked them, Act 5:28 Saying, Did not we straitly command you that ye should not teach in this name? and, behold, ye have filled Jerusalem with your doctrine, and intend to bring this man's blood upon us. Act 5:29 Then Peter and the other apostles answered and said, We ought to obey God rather than men. Act 5:30 The God of our fathers raised up Jesus, whom ye slew and hanged on a tree. Act 5:31 Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins. Act 5:32 And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him. Act 5:33 When they heard that, they were cut to the heart, and took counsel to slay them.
But when some came in the name of James, the Lord's brother (though James had not really sent them) Peter became disobedient to the heavenly vision permitting him to eat with gentiles:
KJV Act 10:28 And he said unto them, Ye know how that it is an unlawful thing for a man that is a Jew to keep company, or come unto one of another nation; but God hath shewed me that I should not call any man common or unclean. Act 10:29 Therefore came I unto you without gainsaying, as soon as I was sent for: I ask therefore for what intent ye have sent for me?
Paul says he was "fearing them which were of the circumcision". Apparently these were the Jerusalem Jews who believed in Jesus the messiah.
So why was he (and Barnabas?) so afraid of them?
Update
I have received several excellent answers (thank you all!) and it appears to me that the answer to the question and the value of the posts depend on the interpretation of the word "fear". In order to facilitate resolving what Paul meant by "feared" I'm posting the BDAG entry in which I have highlighted the section that BDAG assigns to Galatians 2:12:
φοβέω (φέβομαι ‘flee in terror’; Hom. et al.; Wsd 17:9; Jos., Ant. 14, 456), in our lit. only pass. φοβέομαι (Hom.+; OGI 669, 59; SIG 1268 II, 17; pap, LXX, pseudepigr., Philo, Joseph., Just.; Mel., P. 98, 746 al.; Ath. 20, 2; R. 21 p. 75, 1) impf. ἐφοβούμην; 1 fut. φοβηθήσομαι; 1 aor. ἐφοβήθην (Plut., Brut. 1002 [40, 9]; M. Ant. 9, 1, 7; Jer 40:9; Jos., C. Ap. 2, 277; s. B-D-F §79).
① to be in an apprehensive state, be afraid, the aor. oft. in the sense become frightened
ⓐ intr., abs. (Iren. 1, 4, 2 [Harv. I 36, 4]) ἐφοβήθησαν σφόδρα they were terribly frightened (Ex 14:10; 1 Macc 12:52) Mt 17:6; 27:54. ἐπεστράφην φοβηθείς I turned around in terror Hv 4, 3, 7.—Mt 9:8; 14:30; 25:25; Mk 5:33; Ac 16:38. ἐφοβοῦντο γάρ for they were afraid Mk 16:8 (Mk 16:9–20 is now rarely considered a part of the original gospel of Mk, though many scholars doubt that the gosp. really ended w. the words ἐφ. γάρ. The original ending may have been lost; among the possible reasons given are the accidental loss of the last page of Mark’s own first copy [the same defect, at a very early stage, in the case of the 18th book of the Κεστοί of Jul. Africanus: WBauer, Orthodoxy etc. (Engl. tr. of 2d German ed. ’64) ’71, 159ff. S. also FKenyon, Papyrus Rolls and the Ending of St. Mk: JTS 40, ’39, 56f; CRoberts, The Ancient Book and the Ending of St. Mk: ibid. 253–57] or by purposeful suppression, perh. because it may have deviated fr. the other accounts of the resurrection [for the purposeful omission of the end of a document cp. Athen. 4, 61, 166d on the 10th book of Theopompus’ Philippica, ἀφʼ ἧς τινες τὸ τελευταῖον μέρος χωρίσαντες, ἐν ᾧ ἐστιν τὰ περὶ τῶν δημαγωγῶν. S. also Diog. L. 7, 34: a report of Isidorus of Pergamum on the systematic mutilation of books in the library there by Athenodorus the Stoic].—Those who conclude that nothing ever came after ἐφ. γάρ must either assume that the evangelist was prevented fr. finishing his work [Zahn et al.], or indeed intended to close the book w. these words [s. γάρ 1a]. For a short sentence, composed of a verb + γάρ s. also Epict. 3, 9, 19; 4, 8, 4; Artem. 4, 64; 1, 33 p. 35, 6; Plotinus, Ennead 5, 5, a treatise ending in γάρ [PvanderHorst, JTS 23, ’72, 121–24]; Musonius Rufus, Tr. XII; Oenomaus in Eus., PE 6, 7, 8; Libanius, Or. 53 p. 65, 20 F.; PMich 149 VI, 37 [II a.d.]. Among those favoring an ending w. γάρ: Wlh., Loisy, Lohmeyer ad loc.; ABauer, WienerStud 34, 1912, 306ff; LBrun, D. Auferst. Christi 1925, 10ff; OLinton, ThBl 8, 1929, 229–34; JCreed, JTS 31, 1930, 175–80; MGoguel, La foi à la résurr. de Jésus ’33, 176ff; HMosbech, Mkevangeliets Slutning: SEÅ 5, ’40, 56–73; WAllen, JTS 47, ’46, 46–49 [‘feel reverential awe’]; ibid. 48, ’47, 201–3. S. also EGoodspeed, Exp. 8th ser., 18, 1919, 155–60; reconstruction of the ‘lost’ ending, in Engl., by Goodsp. in his Introd. to the NT ’37, 156; HProbyn, Exp. 9th ser., 4, 1925, 120–25; RKevin, JBL 45, 1926, 81–103; MEnslin, ibid. 46, 1927, 62–68; HCadbury, ibid. 344f; MRist, ATR 14, ’32, 143–51; WKnox, HTR 35, ’42, 13ff; EHelzle, Der Schluss des Mk, ’59, diss. Tübingen; FDanker, CTM 38, ’67, 26f; JLuzarraga, Biblica 50, ’69, 497–510; KAland, MBlack Festschr., ’69, 157–80, NTEntwürfe, ’79, 246–83). φοβοῦμαι μᾶλλον I am all the more fearful IPhld 5:1. μὴ φοβηθῆτε do not be afraid Mt 10:31 v.l. (μή 1cεא). μὴ φοβοῦ, μὴ φοβεῖσθε you must no longer be afraid, stop being afraid (μή 1cγא) Mt 10:31; 14:27; 17:7; Mk 5:36; Lk 1:13, 30; 2:10; 5:10; 8:50; 12:7 al. LKöhler, D. Offenbarungsformel ‘Fürchte dich nicht!’: SchTZ 36, 1919, 33ff.—W. acc. of inner obj. (B-D-F §153; Rob. 468; Pla., Prot. 360b; Ael. Aristid. 30 p. 586 D.: φοβοῦμαι φόβον; Did., Gen. 230, 1; on LXX usage s. Johannessohn, Kasus 73) ὁ φόβος ὃν δεῖ σε φοβηθῆναι the fear which you must have Hm 7:1c. ἐφοβήθησαν φόβον μέγαν (Jon 1:10; 1 Macc 10:8; TestAbr. B 13 p. 117, 17f [Stone p. 82]; JosAs 6:1) they were very much afraid Mk 4:41; Lk 2:9. If the nouns are to be taken in the pass. sense, this is also the place for τὸν φόβον αὐτῶν (objective gen.) μὴ φοβηθῆτε 1 Pt 3:14 (cp. Is 8:12) and μὴ φοβούμεναι μηδεμίαν πτόησιν vs. 6 (πτόησις 2); s. 1bγ below.—A recognizable Hellenic expr. (cp. ὁ ἀπὸ τῶν πολεμίων φόβος=fear in the face of the enemy), though encouraged by OT usage (Lev 26:2; Dt 1:29; Jer 1:8, 17; Jdth 5:23; 1 Macc 2:62; 8:12; En 106:4; Helbing 29; B-D-F §149; Rob. 577) φοβ. ἀπό τινος be afraid of someone Mt 10:28a; Lk 12:4; 1 Cl 56:11 (Job 5:22).—Foll. by gen. absol. 56:10. Foll. by μή and the aor. subj. to denote that which one fears (Thu. 1, 36, 1; Aesop, Fab. 317 H.=356a P.; Alex. Aphr. 31, II/2 p. 203, 20 τὸν Ἀπόλλω φοβεῖσθαι μή τι παρελθῇ τούτων ἄπρακτον=Apollo is concerned [almost as much as ‘sees to it’] that nothing of this remains undone; Jos., Ant. 10, 8, Vi. 252) Ac 23:10; 27:17; ITr 5:1; Hs 9, 20, 2. Foll. by μήποτε (Phlegon: 257 Fgm. 36, 2, 4 Jac. p. 1172, 30 φοβοῦμαι περὶ ὑμῶν, μήποτε; JosAs 7:3; ApcMos 16 al.): Hm 12, 5, 3. φοβηθῶμεν μήποτε δοκῇ τις Hb 4:1; μήπου (v.l. μήπως; ParJer 5:5) Ac 27:29; 2 Cor 11:3; 12:20. A notable feature is the prolepsis of the obj. (cp. Soph., Oed. R. 767; Thu. 4, 8, 7) φοβοῦμαι ὑμᾶς μήπως εἰκῇ κεκοπίακα εἰς ὑμᾶς I am afraid my work with you may be wasted Gal 4:11 (B-D-F §476, 3; Rob. 423).—W. inf. foll. be afraid to do or shrink from doing someth. (B-D-F §392, 1b.—X., An. 1, 3, 17 al.; Gen 19:30; 26:7; ApcMos 10:18) Mt 1:20; 2:22; Mk 9:32; Lk 9:45; 2 Cl 5:1.—φοβεῖσθαι abs. in the sense take care (Just., D. 78, 4) πλέον φοβεῖσθαι be more careful than usually ITr 4:1.
ⓑ trans. fear someone or someth.
α. pers. τινά someone (X., An. 3, 2, 19 al.; PGM 4, 2171; Num 21:34; Dt 3:2; Jos., Ant. 13, 26; Just., D. 83, 1) μὴ φοβηθῆτε αὐτούς Mt 10:26. Ἡρῴδης ἐφοβεῖτο τὸν Ἰωάννην Mt 6:20. τοὺς Ἰουδαίους J 9:22.—Gal 2:12; 2:5b (saying of Jesus). God (Did., Gen. 64, 15; Theoph. Ant. 1, 14 [p. 92, 11]) Mt 10:28b; Lk 12:5abc; 23:40; 2:5c (saying of Jesus). The crowd Mt 14:5; 21:26, 46; Mk 11:32; 12:12; Lk 20:19; 22:2; Ac 5:26 (foll. by μή). τὴν ἐξουσίαν (ἐξουσία 5a) Ro 13:3. The angel of repentance Hm 12, 4, 1; Hs 6, 2, 5. The Christian is to have no fear of the devil Hm 7:2a; 12, 4, 6f; 12, 5, 2.
β. animals (in imagery) μὴ φοβείσθωσαν τὰ ἀρνία τοὺς λύκους 2:5a (saying of Jesus, fr. an unknown source).
γ. things τὶ someth. (X., Hell. 4, 4, 8 al.; En 103:4; ApcEsdr 7:2 τὸν θάνατον; Just., D. 1, 5 κόλασιν; Ath., R. 21 p. 75, 1 οὐδέν; Jos., C. Ap. 1, 90; 2, 232) τὸ διάταγμα τοῦ βασιλέως Hb 11:23. τὸν θυμὸν τοῦ βασιλέως vs. 27. τὴν κρίσιν 2 Cl 18:2. τὸν ὄντως θάνατον Dg 10:7. φοβοῦμαι τὴν ὑμῶν ἀγάπην, μὴ … IRo 1:2. τὰ ὅπλα (in imagery) Hm 12, 2, 4.—1 Pt 3:14 and 6 belong here if the nouns in them are to be taken in an act. sense; s. 1a above.—Fear, avoid, shun τὶ someth. (Ps.-Callisth. 1, 41, 9 Δαρεῖος τὸ ἅρμα φοβηθείς) τὴν πλάνην τῶν ἁμαρτωλῶν B 12:10. τὰ ἔργα τοῦ διαβόλου Hm 7:3ac.—AVStröm, Der Hirt des Hermas, Allegorie oder Wirklichkeit? Ntl. Sem. Uppsala 3, ’36.
② to have a profound measure of respect for, (have) reverence, respect, w. special ref. to fear of offending
ⓐ God: fear (differently 1bα) in the sense reverence (Aeschyl., Suppl. 893 δαίμονας; Isocr. 1, 16 τοὺς μὲν θεοὺς φοβοῦ, τοὺς δὲ γονεῖς τίμα; Pla., Leg. 11, 927a; Lysias 9, 17; 32, 17; Plut., De Superstit. 2, 165b; LXX; PsSol 4:21; TestJob 43:9 [τὸν κύριον]; JosAs 2:5 [deities]; Philo, Migr. Abr. 21 [after Gen 42:18]. Cp. PTebt 59, 10 [II b.c.] φοβεῖσθαι καὶ σέβεσθαι τὸ ἱερόν) Lk 1:50 (anticipates the οἱ φοβούμενοι in Ac: H-JKlauck, NTS 43, ’97, 134–39); 18:2, 4 (was Ex 23:1–3 his motto: even God could not bribe him?); Ac 10:35; 1 Pt 2:17; Rv 14:7; 19:5; 1 Cl 21:7; 23:1; 28:1; 45:6; B 10:10f (τὸν κύριον); 19:2, 7; Hm 1:2; 7:1, 4f; Hs 5, 1, 5; 8, 11, 2; D 4:10. Also τὸ ὄνομα τοῦ θεοῦ (2 Esdr 11) Rv 11:18.—φοβούμενοι τὸν θεόν as a t.t.=σεβόμενοι τὸν θεόν (σέβω 1b; t.t. disputed by MWilcox, JSNT 13, ’81, 102–22; cp. TFinn, CBQ 47, ’85, 75–84; ILevinskaya, The Book of Acts in Its Diaspora Setting [BAFCS V] ’96, 51–126; BWander, Gottesfürchtige und Sympathisanten [WUNT 104] ’98, esp. 80–86; 180–203) Ac 13:16, 26 (Just., D. 10, 4 al.; sing. 10:2, 22).—τὸν κύριον (PsSol 2:33; 3:12 al.; JosAs 8:9) Christ: Col 3:22.—WAllen (s. 1a above) interprets Mk 16:8 to mean reverence for the divine.
ⓑ pers. who command respect (Plut., Galba 1054 [3, 4]; Herodian 3, 13, 2; Lev 19:3 φοβ. πατέρα καὶ μητέρα; Jos., Ant. 19, 345): of a wife ἵνα φοβῆται τὸν ἄνδρα Eph 5:33. τὸν ἐπίσκοπον IEph 6:1.—RAC VIII 661–99; TRE XI 756–59; Schmidt, Syn. III 507–36. DELG s.v. φέβομαι II. M-M. EDNT. TW. Sv.
BDAG
I might as well include the LSJ lexicon entry as well:
http://logeion.uchicago.edu/%CF%86%CE%BF%CE%B2%CE%AD%CF%89
Update
Since posting this question and receiving an accepted I came across this excellent exposition of the situation in Galatia.