Prevent the first Great Awakening at all costs.

Have deism and strict banning of all religious symbols in government absolutely corner-stoned into the fledgling United States.

I don’t know how to do this, maybe even carry over Jacobin ideas during the French Revolution. But the seeds to basically leave organized religion, especially Christianity in the liturgical form (maybe not the idea of God) are there among the founding fathers and the intellect of the time across the Western World. It just didn’t stick but it could given the right nudges.
 
Rather than increasing migration from non-Christian areas, you could also decrease migration from Christian areas - namely Catholic ones. If the USA retains an anti-Catholic stance for longer, we could see waves of Catholic migrants instead heading for Latin America and Canada, strengthening those countries relative to the USA and creating a positive feedback loop where migrants prefer other destinations. This may result in a western USA less settled by whites, allowing for a larger native population who may remain largely non-Christian. Even if the Americans later relax their immigration policy, they will have missed out on the major waves of Catholic migration and will instead see people coming mostly from Asia - people who are mostly not Christian.
 
Perhaps early pogroms in Eastern Europe, combined with no Chinese exclusion act? The end result being that the country is about 5% Jewish and 5% Buddhist or traditional Chinese religion.
 
It seems like the Chinese option got the upper hand. Now the question is how big would the Chinese non-Christian community be by today?

I don’t know about this, my asian friends are often very Christian

At any rate, preventing immigration in general from about 1790 on, would still have a US of a comparable population size. However, maybe if you had a similar opinion change as in New England happen everywhere. It went from being one of the most religious to least religious areas in America in about 2 centuries.
 
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