The cardinal thought of Jonathan Mayhew with reference to religion was that of free inquiry. Diligent and free examination of all questions, he felt, was necessary to any acquisition of the truth. He believed in liberty and toleration everywhere, and this made him accept in the fullest sense the doctrine of the freedom of the will. In man he found a self-determining power, the source of his moral and intellectual freedom. He said that we are more certain of the fact that we are free than we are of the truth of Christianity. This belief led him to the rejection of the Calvinistic doctrine of inability, and to a strong faith in the moral and spiritual possibilities of human nature. He described Christianity as "a practical science, the art of living piously and virtuously."[6] He had quite freed his mind from bondage to creeds when he said that, "how much soever any man may be mistaken in opinion concerning the terms of salvation, yet if he is practically in the right there is no doubt but he will be accepted of God."[7] He held that no speculative error, however great, is sufficient to exclude a good and upright man from the kingdom of heaven, who lives according to the genuine spirit of the gospel. To him the principle of grace was always a principle of goodness and holiness; and he held that grace can never be operative as a saving power without obedience to that righteousness and love which Christ taught as essential.[8] He declared that "the doctrine that men may obtain salvation without ceasing to do evil and learning to do well, without yielding a sincere obedience to the laws of Christianity, is not so properly called a doctrine of grace as it is a doctrine of devils."[9] He said, again, that we cannot be justified by a faith that is without obedience; for it is obedience and good works that give to faith all its life, efficacy, and perfection.[10] [Sidenote: The First Unitarian.] Dr. Mayhew accepted without equivocation the right of private judgment in religion, and he practised it judicially and with wise insight. He unhesitatingly applied the rational method to all theological problems, and to him reason was the final court of appeal for everything connected with religion. His love of freedom was enthusiastic and persistent, and he was zealously committed to the principle of individuality. He believed in the essential goodness of human nature, and in the doctrine of the Divine Unity. He was the first outspoken Unitarian in New England, not merely because he rejected the doctrine of the Trinity, but because he accepted all the cardinal principles developed by that movement since his day. He was a rationalist, an individualist, a defender of personal freedom, and tested religious practices by the standard of common sense. His sermons were plain, direct, vigorous, and modern. A truly religious man, Mayhew taught a practical and humanitarian religion, genuinely ethical, and faithful in inculcating the motive of civic duty.