Paul's statement in Galatians 3:28 has been cited to support sundry erroneous positions, but it is typically misinterpreted in one of two ways. The first is to misread Paul as if he’s saying all human distinctions are obliterated. The second is to underrealize the significance of what Paul is saying.
First, some people appeal to Galatians 3:28 to defend egalitarianism, transgenderism, comprehensive cultural assimilation, ethnic indifference, classless societies, and more. But those ideas are irrelevant to Paul’s point. The parity he’s advocating is with respect to salvation; it’s not a wholesale blurring of all human distinctions. In fact, certain distinctions were established at creation—such as between the Sabbath and ordinary days (Gen. 2:2–3; Ex. 16:22–26; Mark 7:19), between labor and rest (Gen. 2:15; 2 Thess. 3:10; James 5:4), and in gender roles (Gen. 2:18; 1 Cor. 11:3–16). This reality informs the general abiding validity of the three categories Paul mentions: gender roles (male-female), socio-labor distinctions (slave-free), and ethnicity (Jew-Greek).