Members of Thrivent Financial for Lutherans start casting votes Friday on whether to open membership to all Christians, not just Lutherans — a decision that could profoundly affect a company that blends faith and finance.

The two-month voting process kicks off as Minneapolis-based Thrivent racks up another strong year, aided by rallying stock market prices. The not-for-profit membership organization doesn’t report profits per se but said Wednesday that its total surplus for 2012, an accumulation of internally generated earnings, climbed 13 percent last year to an all-time high of $6.1 billion.

It was Thrivent’s strongest growth since 2006, and the company said it was one of the best years financially in its 111 years.

Strong results in Thrivent’s life insurance and annuities operation, its main financial engine, and good returns on surplus invested in equities and other vehicles fired the growth.

Assets under management grew 8.4 percent to $82.2 billion and revenue rose 4 percent to $8.3 billion. Sales of life insurance and variable annuities rose 14 and 18 percent, respectively.

Lutheran congregations, meanwhile, have not necessarily been growing. And Thrivent has been exploring for some time ways to broaden its mission of strengthening Christian communities by helping people be “wise with money” and “live generously.”

To qualify as a tax-exempt fraternal benefit society, its membership must have some sort of common bond. Thrivent’s board decided last year to put the idea of expanding the common bond from Lutheran to Christian to a majority vote.

Starting Friday, about 2.2 million Thrivent members who are benefit members will vote on the question by mail, phone or online. Benefit members are at least 16 and have at least one life, health or annuity contract with Thrivent.

A decision will be made by early summer. Changes won’t be immediate.

A quick scan of Thrivent’s Facebook page suggests there are plenty of members who want to keep the organization Lutheran-only, and that some members feel Thrivent’s leaders have advocated too strongly for expansion.

In an interview, Thrivent CFO Randy Boushek said he’s optimistic a majority will vote for expansion. “It will be a very slow transformation of our business,” Boushek said.

“We have a very unique value proposition in the marketplace,” he said. “There is a strong need for that.”