Christian volunteer work offers by far the most opportunities for an international volunteer mission.
Many of these organizations have a long history of helping overseas and although Christian-based, not all of them require you to practise your faith openly.
The Catholic Network of Volunteer Service, or CNVS, is relatively broad in its admissions policy. A number of their programs don't require university degrees, nor is age a major issue, whether young or old.
Length of service is flexible, from weeks to several years, and they operate in more than 100 countries worldwide.
While CNVS is faith-based and primarily Roman Catholic, this is not a prerequisite. Each program is different - some require religious activity, others none at all.
Prospective volunteers wishing to work in the United States could consider the Lutheran Volunteer Corps, or LVC, active in ten or so inner cities of America. Much will depend on your ability to obtain a one-year visa to the US, but this group does not require American citizenship and is non-denominational.
In its own words, LVC is "for people who want to explore their spirituality while working for social justice, living in intentional community with other volunteers, and simplifying their lifestyles." It is also one of the few faith-based groups that doesn't discriminate on the basis of sexual orientation, making it an attractive alternative for lesbians of faith. Basic living expenses are taken care of, and a small stipend provided.
One group with extensive opportunities for Christian volunteer work is the Mennonite Central Committee, or MCC, which sends volunteers on both development and humanitarian assignments. It asks that volunteers be practising Christians, though not necessarily members of the Mennonite Church, and its lifestyle requirements are relatively strict.
Most volunteers are from the US or Canada but other nationalities are welcome if recommended by a national Mennonite church. MCC pays all volunteer expenses, including transportation, health insurance and education of volunteers' children. It also provides a small stipend for extra expenses every month.
Another Christian group with significant opportunities is the United Methodist General Board of Global Ministries, or GBGM, which requires Christian faith (and in some cases Methodist membership). This is a large program and activities and countries are varied, lasting from a few weeks to a few years.
Another large program is Habitat for Humanity, whose volunteers help build houses for needy families over one to three weeks. It is non-denominational but does not cover your costs - you'll have to fundraise.
Plenty of Christian volunteer work exists with the Quakers, or Friends. The American branch, called the American Friends Service Committee, is a Quaker organization which includes people of various faiths who are committed to social justice, peace, and humanitarian service. Being a Quaker is not a prerequisite for volunteers.
They have several types of international volunteer mission, including including work camps in Africa and Peace Brigades International.
Another Christian faith-based group not requiring religious affiliation is the Brethren Volunteer Service, or BVS, which places Americans and Canadians abroad, or provides an international volunteer mission in the US for volunteers from other countries. They also cover most costs, although there is a program fee for international placements.
BVS has a wide range of Christian volunteer work opportunities - with children, refugees, disabled persons, farmers, the homeless, and more.
The Jesuit Volunteers International, while Catholic-based, doesn't require its volunteers to be Catholic, or even Christian. Mostly the JVI places volunteers who teach a number of subjects, and who take part in group activities and prayer and meditation, which are expected even of non-Christians.
What if you're not a Christian? No problem - there are plenty of other faith based volunteering opportunities if this type of volunteering is what you see.