Sarah conceived and bore a son to Abraham in his old age, at the set time of which Hashem had spoken. (21:2)
We are aware that there is a prohibition to call Avraham by the name Avram. The Ohr Hachaim asks why isn't there a prohibition to call Yisroel by the name Yaakov or Sarah by the name Sarai?
The Ohr Hachaim explains that the answer lies in this posuk. The child that Sarah bore to Avraham is called 'the son of Avraham.' Furthermore, Hashem told Avraham that only in Yitzchok would his legacy endure. Yishmael was also born to Avraham but he was born before Hashem changed his name from Avram to Avraham. Yishmael was the son of Avram not the son of Avraham. In summary, Yishmael is the son of 'Avram,' whereas Yitzchak is the son of 'Avraham.' When a person calls Avraham by the name Avram he is, in essence, declaring that the primary progeny of our patriarch is Yishmael. This is patently false and a dangerous deviation from an authentic Torah Hashkafa. However, this is not so concerning a change from Yisroel and Sarah. True that it may be inaccurate or misunderstood out of context, but those deviations do not have the same cosmic consequences and ramifications.
