How does one respond to secularists who claim that the hijab was not mandated, citing that Umar used to strike slave women if they wore the hijab and say to them: "Do not liken yourselves to free women"?

The hijab is an obligation from Allah in the Quran and Sunnah, not a decree of rulers. Among the proofs are His (Allah's) saying: "And do not display yourselves as [was] the display of the former times of ignorance" (Al-Ahzab: 33), and His saying: "and to draw their headcovers over their chests" (An-Nur: 31), and His saying: "to bring down over themselves of their outer garments" (Al-Ahzab: 59), and the Prophet's (peace be upon him) saying: "The woman is 'awrah (i.e., her entire body should be covered)."

The women of the Pious Predecessors (Salaf) adhered to the hijab, as narrated from Aisha (may Allah be pleased with her). As for the difference between slave women (Ima’) and free women (Hara’ir), slave women were permitted to uncover their faces due to their need for service, provided that it did not cause temptation. However, for free women, the hijab is obligatory upon them.