'Diversity is our strength' poison pill

I'm trying to answer a family member who is liberal and supportive of leftist causes, but not due to the typical reasons of leftists - ie envy, seeing world as oppressor vs oppressed, or just being a complete loser. Instead his view is derived from his interpretation of the intent of the country - a beacon for all, etc.

I don't think this is what the Founding Fathers intended. They'd roll in their graves if they saw what is happening here and in Europe. Best I can come up with is John Adams - “Our Constitution was made only for a moral and religious People. It is wholly inadequate to the government of any other.” And this is already a strong argument. It informs us that you must have a consensus on 'right vs wrong' if you expect a Constitutional Republic to work out. Also I don't think freedom of religion was an open endorsement that diluting say Europe with Muslims was advisable, and so I think that concept gets overused. (Funny, those who do that don't get so excited about the rest of 1A or 2A at all).

At some point the poison pill of 'diversity is our strength' entered our consciousness and I don't think the FF's believed our strength was diversity. So I'm looking for more historical evidence to prove this. Thanks in advance