Rashi: He built a Mizbe'ach, and reminded Yisrael that Mizbach Hashem should be on their hearts and fluent in their mouths, for it was destroyed, idle, and its name and mention had ceased among all 10 Shevatim.

Rashi citing a Midrash: Sha'ul built a Mizbe'ach in Har ha'Karmel - "Ba Sha'ul ha'Karmelah v'Hinei Metziv Lo Yad" (Shmuel I, 15:12) and it says "va'Yatzev Sham Mizbe'ach" (Bereishis 33:20). Malchei Yisrael destroyed all the Mizbechos and Bamos in their land that were l'Shem Shamayim ¹ . Eliyahu built Sha'ul's Mizbe'ach, which had been destroyed. Radak - this is a borrowed expression, to call rebuilding something destroyed 'Refu'ah.' We find "Alsah Aruchah l'Chomos Yerushalayim" (Nechemyah 4:1) and "v'Yo'av Yechayeh Es She'ar ha'Ir" (Divrei ha'Yamim I, 11:8).

Radak: Through the miracle via Eliyahu, Yisrael repented and offered Korbanos on Mizbach Hashem that had been destroyed and withheld from them.

Malbim: Eliyahu had built a Mizbe'ach immediately [after they agreed to the test], and Nevi'ei ha'Ba'al destroyed it, for they thought that it ruined their Avodah. He now returned it to its strength.