An essay, to defend some of the most important principles in the Protestant Reformed system of Christianity, on which the churches of Christ in New-England, were originally founded more especially; the doctrine of Christ's sacrifice and atonement, as being absolutely necessary to the pardon of sins, consistently with God's infinite and unchangeable rectitude; against the injurious aspersions cast on the same, by Jonathan Mayhew, D.D. in his late thanksgiving sermons on Psal. cxlv. 9. In which some of the doctor's mistakes, inaccuracies and inconsistencies, are pointed out.