Q: To teach English overseas should I get a masters degree or a certificate?

When I began as a tentmaker missionary in the mid 1980s I found that much of the world was open to anyone who had a bachelor's degree in any subject and was a native English speaker. Some places want a certificate or degree in Teaching English as a Second Language (TESL). Generally speaking, to teach in a language school the CELTA certificate is the way to go. If you want to teach in a university, a Masters degree in TESL or Linguistics may be best. I served in Japan and worked for the largest language teaching company in Japan. I loved evangelizing my businessmen students. Meanwhile, there were missions agencies ministries that used TESL for church planting. To my knowledge you do not need much TESL training to join them.

Many of us who teach English have absolutely no credentials other than being native English speakers. Of course, knowing HOW to teach would be a big help, but a master's degree is probably just overkill, especially since teaching English will be your second job.

This depends most on where you want to teach. In many places just a certificate will do. The more developed a country is, the more certification it will want to see in expatriate teachers. The Masters Degree will give you more versatility (and probably equip you to be a better teacher), but if you know where you want to go and if they accept a certificate, why spend the extra years in school?

In the Gulf, if you want to teach in a university, you've got to have an M.A. One good thing about teaching in a university is your teaching load of only 20 hours per weeek and teaching during the day. If you don't have an M.A., you might be teaching 30 hours per week and most of them in the evenings. As a church planting tentmaker, you'll have more time to spend with your students teaching in a university. I taught 12 years in a university and one year in a language institute in the Arabian Peninsula.