They form an exclusive sorority. Some were impoverished and some rich, some were unlettered and some college graduates.

They were all mothers of U.S. presidents.

Most shared one trait: mild to fervent commitment to the Christian religion, often buttressed by daily Bible reading and weekly worship.

But not all. Apparently some of their sons who became president outdid them in religious fervor, or at least commitment.

Harold I. Gullan tells what we know about the beliefs of the presidential moms in the popular history “Faith of Our Mothers: The Stories of Presidential Mothers from Mary Washington to Barbara Bush.”

Because the record is sometimes thin, Gullan fills out the proceedings with other biographical material, including another trait most of these mothers shared: unshakable faith in their sons’ destinies.

Gullan doesn’t come out and say so, but several recent presidents surpassed their mothers in religious zeal, three Bible Belters in particular.

Methodist George W. Bush, who experienced a midlife recommitment to Jesus Christ, expresses more fervor than mother Barbara, an Episcopalian.

Bill Clinton has been a committed Baptist since turning up at a local church at age 10 and informing the preacher he’d be coming to worship regularly, with or without mother Virginia Dell Cassidy Blythe Clinton Dwire Kelley (she was married four times).

Jimmy Carter added “born again” to the political lexicon and has taught Bible classes for decades, while mother “Miss Lillian” seemed a less robust style of Baptist.

Then we have the reverse, mothers whose degree of piety was not passed along with their genes.

These included the only Roman Catholic among the moms, Rose Kennedy. Her 104 years were a regimen of devotionals and Masses (often daily). Mrs. Kennedy was so old-school that she refused to attend the funeral of daughter Kathleen, who had married a Protestant divorcee.

By Gullan’s account, Catholicism gave Mrs. Kennedy a refuge from her husband’s flagrant philandering. But she was able to pass on the faith “to her children only in part.”

A Protestant version of the piety gap is seen in Warren G. Harding and mother Phoebe. The mother’s “second home” was the Methodist church, Gullan recounts, and she became even more devout as a convert to Seventh-day Adventism.

Son Warren affiliated with the Baptists but displayed little piety and died in moral disgrace in the White House. Upon his mother’s death in 1910 he remarked that he could not say this for himself “but dear, dear Mother will wear a crown” of God in eternity.

The enigmatic Abraham Lincoln revered his stepmother Sarah (who is in the book along with Lincoln’s birth mother). Sarah could not read but imparted a love of learning and of the Bible. Yet Lincoln stayed at home when his parents worshipped with the Baptists and he’s believed to be the only president who never joined a church.

Two of the mothers failed to pass on the most distinctive tenet of their creed.

Hannah Nixon and her husband raised son Richard M. as a Quaker. Theirs was the Western evangelical type, quite distinct from the quiet version of the Northeast, but both branches forbade the bearing of arms. Yet Richard fought in World War II and became commander in chief.

So did Dwight D. Eisenhower, whose father and mother Ida were devout followers of a “peace church,” the Brethren in Christ (aka “River Brethren”). When Ike was an adult, the elder Eisenhowers converted to the Jehovah’s Witnesses, who oppose participation not only in the military but voting or anything else to do with what they see as a wicked worldly system.

Many of the mothers were related to preachers. The most distinguished example was the husband of Woodrow Wilson’s mother Janet (“Jessie”), who became a seminary professor and the chief officer of what is now called the Presbyterian Church (U.S.A.)