If Baptism does no more than welcome an infant into the fold, we might as well use confetti, earnest felt banners, and a cake.  The belief you hold is neither Scriptural nor Traditional and makes no sense in either context.  
I'm sorry but, if Jesus came to reform Judaism he was a terrible reformer and utterly miserable teacher - certainly not God - who chose to, very effectively, speak through prophets in earlier times.  It wasn't merely the "temple elites" who rejected His teaching but whole swaths of the Jewish community, both in and out of Palestine.
Jesus came to establish the Kingdom of God on Earth.  The Church (both hierarchically and as a people) are both the cause of its spread and the effective remedy for our separation from God.
Why would water, ineffective on an infant, be efficacious on an adult in the same way as acknowledging sin?  Jesus became man, not merely to have us serve God better, but "that man might become god".