Analogy of cultural life: the evolution and incredible diversity of human expression in no way invalidates the fact that humanity constitutes one “human race”.
A similar evolutionary process has characterized humanity’s religious life. But rather than simply representing history's ongoing method of trial and error, such norms were explicitly prescribed in each case.
While certain features of each code of conduct would eventually fulfil their purpose and be overshadowed by concerns of a different nature, the code itself would lose none of its authority.
To argue that differences in teachings represent any significant objection to the idea of revealed religion's essential oneness is to miss the purpose that these prescriptions served.
The eternal laws open the way for the soul to enter into an evermore mature relationship with its Creator. This is not at all irreconcilable with the transitory laws providing auxiliary guidance that enhances the process of civilization building.