Rufus King (1755-1827) was a signer of the Constitution and an important early American politician. As a member of the Continental Congress he supported the resolution to call for a Constitutional Convention, and later served both as a United States Senator, and as an Ambassador to Great Britain.
Despite his importance as an American founder, King rarely spoke or wrote about his views on church and state. What evidence there is, however, shows that King, in keeping with his Massachusetts upbringing, had few problems with state aid to religion. As a member of the Continental Congress, for example, King supported a provision in an early draft of the Northwest Ordinance that would have set aside land in the Northwest Territory for the support of religious organizations (this provision was ultimately rejected by Congress). Similarly, King supported the provision in the 1821 Massachusetts Constitution that provided aid to ministers of the Gospel.
Alas, neither King's work on the Northwest Ordinance, nor his support of Christianity in Massachusetts, tells us anything about his views concerning church and state under the Constitution. As we note elsewhere in this site, the Northwest Ordinance was written under the Articles of Confederation, not the Constitution, and there is no way of knowing if King thought that a religious aid provision would have been legal under the First Amendment. Similarly, aid to religion by a state is not the same as aid to religion by the federal government. So far as we know, King said nothing to indicate that he would support federal aid to religion in the states. On the contrary, according to King biographer Robert Ernst, King was a Madisonian who believed that Congress was limited to the delegations of power contained in the Constitution. Since there is no provision that allows religious aid to the states, it's doubtful that he would have accepted anything like the modern doctrine of accomodationism.
Finally, we note that King was at least somewhat skeptical of alliances between church and state. Ernst, for example, notes that, despite the fact that King was an Episcopalian,
Hence, while King was sympathetic to the idea that Christianity was the preferred religion of the state of Massachusetts, he didn't seem to want Christianity to exercise control of government. On the contrary, his hope was that the church would never establish a religious hierarchy on American soil.