Actually the Founders who insisted on secularism were not all Christian.
They were not all theological christians. But if you regard Christian as "one who believes in the Christian values" - then every founder was Christian.
And the nations doing better than the US today have little to no religion in the public sector period.
Wait, are you saying USA today is better today? NO! The USA was the best country to live in, period. This was true at least the 200 first years following their Christian constitution.
Secularism: the division between church and state, was a Christian idea, though the roman empire destroyed that initial idea by adopting Christianity as state religion after persecuting Christians and killing them for 300 years.