This is nominalism IMO.
It's both a fact of history and theology that the Old Testament Laws were different from the New. Your error is that you assume God dealt with people in these different times in the same way. God created the rules of the Old Law and He provided graces to every human being, based on this Old Law. He wanted all men to be saved in the Old Testament, just like He does in the New. When God created the New Law, He correspondingly provided different graces, based on the new rules.
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At the end of the day, every error related to salvation/baptism (and there are countless errors) comes from a lack of faith in God's all-loving Providence. People think they can understand/explain how God communicates, treats and speaks to every human being in a different way; but this is a theological mystery, as is salvation in general.
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Instead of accepting the biblical truth that God truly, infallibly, and fully "wills all men to be saved",
through the visible Church, men invent ways around this, because otherwise salvation is unexplainable. And modern men can't stand the idea of not knowing something, which is simply, pride.