This links to Fischer's original words. One thing is for sure: George Washington was pro-religion. There is smoking gun evidence for this. There is a world of difference however between "religion" on the one hand, and what Fischer understands to be "Christianity" on the other.
Religion is a genus of which Christianity is a species. And Fischer's Christianity (like for instance President Obama's or the Pope's) is a further subspecies. Fischer's error is that he conflates Washington's genus with Fischer's own subspecies.
President Washington didn't have a problem with the then conservative Christian clergy (the ones who tried to sniff Jefferson out as an "infidel"). However, GW didn't seem to have a problem with any religious sect, provided they weren't Tories and that their faith yielded moral practice.
Given the contentious nature of the debate, we have to draw our conclusions carefully. We know GW believed in a warm Providence and was, as noted, pro-religion. I suspect, for instance, that on the nature of future punishment, he was a universalist. I don't have any smoking gun quotations to prove it for certain. However, he did give John Murray's Universalist Church the highest regards he gave to any other sect. But admittedly many of those other sects that earned his imprimatur were not universalist on the matter.
Likewise with the Trinity, I don't see GW as an "orthodox Trinitarian" Christian. However, he never bitterly ridiculed the orthodox doctrine like Thomas Jefferson and John Adams did. He did give props to an address by the Arian Richard Price which was pro-unitarian. And in that address Price discusses the importance of granting rights to "religion" and not "Christianity."
As Price argues:
From the preceding observations it may be concluded that it is impossible I should not admire the following article in the declaration of rights which forms the foundation of the Massachusett's constitution:'In this state every denomination of Christians demeaning themselves peaceably and as good subjects of the commonwealth shall be equally under the protection of the law, and no subordination of any one sect or denomination to another shall ever be established by law.'This is liberal beyond all example. I should, however, have admired it more had it been more liberal, and the words, all men of all religions been substituted for the words, every denomination of Christians.It appears farther from the preceding observations that I cannot but dislike the religious tests which make a part of several of the American constitutions. In the Massachusett's constitution it is ordered that all who take seats in the House of Representatives or Senate shall declare 'their firm persuasion of the truth of the Christian religion'. The same is required by the Maryland constitution, as a condition of being admitted into any places of profit or trust. In Pensylvania every member of the House of Representatives is required to declare that he 'acknowledges the Scriptures of the Old and New Testament to be given by divine inspiration'. In the state of Delaware, that 'he believes in God the Father, and in Jesus Christ his only Son, and in the Holy Ghost, one God blessed for evermore'. All this is more than is required even in England where, though every person however debauched or atheistical is required to receive the sacrament as a qualification for inferior places, no other religious test is imposed on members of parliament than a declaration against Popery. It is an observation no less just than common that such tests exclude only honest men. The dishonest never scruple them.Montesquieu probably was not a Christian. Newton and Locke were not Trinitarians and therefore not Christians according to the commonly received ideas of Christianity. Would the United States, for this reason, deny such men, were they living, all places of trust and power among them?