One might have once said 'Crusader', but that is so far of the scale of 'incorrect' these days... HighTechGeek's answer in my view sweeps the field, but let me also elevate a term that he noted elsewhere and might add to his answer at a later time: 'Champion'. Firstly from the OED:
champion ▪ I.champion, n.1
(ˈtʃæmpɪən)Forms: 3–4 champiun, 4–5 champioun, (chaumpion, -yon, -youn, scaumpioun, schampion, 5–6 champyon), 4– champion.
[ME. champiun, -on, a. OF. champiun, -on (= Pr. campio, -on, Sp. campion, -eon, Pg. campião, -eão, It. campione):—late L. campio, -ōnem combatant in the campus or arena, professed fighter, f. L. campus field of athletic or military exercise, place of combat, lists: see camp n.1and n.2
- fig. and transf. One who in any kind of contest or conflict acts as the acknowledged defender of a person, cause, or side: one who stoutly maintains any cause. (The literal sense is sometimes distinctly in view, sometimes out of sight.)
a 1300 Cursor M. 18651 Sua did iesus, vr champiun [v.r. scaumpioun], Þof he lai ded for vr ranscun. 138. Wyclif Sel. Wks. III. 289 Strong schampions and pileris of holy chirche. 1483 Caxton Gold. Leg. 430/4 God fader..gafe & betoke the sayd kyng champyon or deffensour of the feythe. 1584 R. Scot Discov. Witchcr. ii. i. 15 Bodin the champion of witchmongers. 1593 Shakes. Rich. II, i. ii. 43 To heauen, the widdowes Champion. c 1665 Mrs. Hutchinson Mem. Col. Hutchinson (1846) 25 He hated persecution for religion, and was always a champion for all religious people against all their great oppressors. 1751 Johnson Rambl. No. 144 ⁋8 No other qualification for a champion of controversy. 1806 Med. Jrnl. XV. 544 The great champion of vaccination. 1841–44 Emerson Ess. Heroism Wks. (Bohn) I. 110 Human virtue demands her champions and martyrs. 1851 Robertson Serm. Ser. ii. i. (1864) 10 He never was the champion of a class, because He was the champion of Humanity.
Hence a 'Champion for Social Justice', or any other cause substituted in the place of Social Justice. Champion to my mind has a particular connotation in that in nearly all historical senses it was a person who fought on behalf of others, and not for him or her self. Of course in sporting parlance 'Champion' is often now the title-winner, but it is interesting how it has not been used as a term describing military success in modern times, perhaps because it was considered too old fashioned and noble, and indicative of good sportsmanship...