This is the theme of ch. 5:13 to 6:15. Love for one’s fellow men is the theme of the last six commands of the Decalogue (see on Matt. 22:39, 40). These six govern the relation of man to man, as the first four govern relations between God and man. Paul’s silence here concerning the first four commands does not imply that they have lost their value for the Christian. Far from it. The theme of the epistle thus far has been right relations between a Christian and his God. But lest the Galatians err in thinking that religion consists only in a right relationship to God, Paul now makes clear that it consists also in right relations with one’s fellow men. The citation is from Lev. 19:18.
The Greek has the definite article (see on Rom. 2:12). Here Paul refers to the torah, that is, to all the revealed will of God to man, but especially to the moral law, as the context implies.