Adam Clarke on Romans 9:18

Therefore hath he mercy on whom he will - This is the apostle's conclusion from the facts already laid down: that God, according to his own will and wisdom, in perfect righteousness, bestows mercy; that is to say, his blessings upon one part of mankind, (the Jews of old, and the Gentiles of the present time), while he suffers another part (the Egyptians of old, and the Jews of the present day) to go on in the abuse of his goodness and forbearance, hardening themselves in sin, till he brings upon them a most just and exemplary punishment, unless this be prevented by their deep repentance and general return to God through Jesus the promised, the real Messiah.