I have heard from cousins on this Davis side of my family, as well as a Quaker researcher in Philidelphia regarding this unpresidented "leap of faith", for lack of a better term.
Apparently Willaim did not see a church as a relgious bastion, in fact he advocated that relegion was societies invention and that G_D was to be seen as faith and belief, rather than pomp and ceremony, a very strong parallel to the Quaker meeting.He was able to "sit" at meetings of other denominations possibly because he treated them equally.
Rev. William was a remarkable man in his own right.At the age of twelve he had a severe falling out of the faith with his parents and other family members.His parents were staunch Episcopalians who were amoung the strongest disenters and persecuters of Quakers.And yet . . .
After the fallout with his family William stayed with a free black gentleman who was a leader of the "Holy Lights", an offshoot of the Baptist church.William eventually went to live with a Baptist family for a few months.Before his 14th birthday he returned to his parents home and was able to convince them they were wrong to persecute others for their faith.
Within days (according to all biographers and family history) William was able to influence his parents, all of his sisters, and all but one of his brothers to renounce the Episcopalian church and become members of the Baptist.He was 13 at the time.
He eventually would attend seminary and in @1786/87 was ordained a minister of the Baptist church.His father and all of his brothers became Deacons or ministers in their own right.Of his sisters they would all marry either Deacons or ministers of the church. Two of his sons-in-law were also ministers.
One of the five members of the Sarepta, Rev Thomas Johnson, was the brother of Mary Johnson who married William's eldest brother James Davis in 1780.