But they ask, What are the Benefits and Advantages of the christian Revelation, if the Heathen World living up to the Light of Nature and Reason may be sav’d? For Answer to this, I refer them to that excellent Defence of Christianity by Mr. Foster, Chap. 1. ...The context here is that Hemphill argued and Franklin agreed "that Christianity, [as to it’s most essential and necessary Parts,] is plainly Nothing else, but a second Revelation of God’s Will founded upon the first Revelation, which God made to us by the Light of Nature." To which the orthodox argued, if Heathens living up to Nature and Reason can be saved (because this -- to be a good moral person -- is what it really means to be a "Christian") even though they have not accepted or ever heard of Christ, then what use is the Gospel?
The orthodox, in this sense, echo the view of the "Christian-Deist" Matthew Tindal who argued what is valuable, useful and true in "Christianity" are only those parts that are IDENTICAL to the law of nature as discovered by reason alone. The parts not identical (i.e., much of orthodox Christian dogma) were the result of superstitious error (what Joseph Priestley would later term "corruption").
Hence, since we can discover what is valuable and true about Christianity through reason unaided by special revelation, the latter is useless and arguably harmful because of the superstitious errors intermixed in the Bible and authoritative ecclesiastical teachings. In other words, even though special revelation contains truth, it's unnecessary and superfluous when reason can figure those things out without it and doesn't have to waste its time sorting through doctrinal errors of religious superstitions based there upon.
The orthodox (of the Catholic, Anglican or Protestant bent), simply disagree with the Tindal's premise of what constitutes religious truth and error. Much of the authoritative church teachings based on special revelation that Tindal tries to sweep away with his "reason" are, to them, non-negotiable doctrines central to their faith. That's why they might be extremely suspicious of anyone who hints at "real Christianity" as largely a "republication" of the law of nature.
Hopefully this sheds contextual light on the title of Tindal's most notable Book of Deism: "Christianity As Old As the Creation: Or the Gospel a Republication of the Religion of Nature" as well as the suspicion the Presbyterians had against Hemphill and Franklin who agreed that the essential parts of Christianity are largely republications of the law of nature, God's first revelation to man.
As it were, Franklin and Hemphill, in their defense relied on Rev.James Foster's book "The Usefulness, Truth, and Excellency of the Christian Revelation Defended" to authoritatively defend special revelation. Note, though, Foster shared the premise that Christianity is largely a republication of the law of nature and that heathens following nature and reason can be saved even if they never consciously accepted Christ.
Their point was, even given it true that Christianity essentially republishes the law of nature as discovered by reason, special revelation is nonetheless useful and true. And, interestingly, because Tindal and Foster shared the given premise (but disagreed on special revelation), their debate was quite friendly, such that Tindal gave his props to Foster's book that challenges his.
Arguably Foster was, like Tindal a "Christian-Deist" and this was a debate between Christian-Deists. Or perhaps this was a debate between "Christian-Deist" Tindal and "Christian-Unitarian" Foster. In any event Foster's book is a liberal unitarian defense of special revelation.
I can't do justice to everything Foster's book says, but I understand the bottom line of its defense of Christianity as follows: Yes, Christianity essentially republishes the law of nature as discovered by reason, but the New Testament does this almost perfectly so. So much so that it operates as a "shortcut" for most ordinary men who don't possess the philosophical acumen of the wisest of philosophers who can figure these things out without the need for special revelation.
In addition, the New Testament contains some improvements made on the law of nature as discovered by reason that are uniquely found there and nowhere else.