I have never in my life heard one scripture that refers to believers as "slaves of God". Perhaps you can quote book, chapter, and verse.
Romans 6:22 (NIV) "But now that you have been set free from sin and have become slaves to God, the benefit you reap leads to holiness, and the result is eternal life."
I don't know Hebrew, but I have a Langenscheidt's Pocket Hebrew Dictionary to the Old Testament (Hebrew/English) which transliterates Obadiah's name as 'obadyahu. The root word is transliterated as 'ebed (spelled with the letters ayin, beth, daleth), with the meanings "labourer," "servant," "slave," "bondsman," "subject"; "worshipper." (The dictionary also uses long and short marks and accents that I can't insert.)
Strong's Greek:
1401 doulos {doo'-los}
from 1210; TDNT - 2:261,182; adj
AV - servant 118, bond 6, bondman 1; 125
1) a slave, bondman, man of servile condition
1a) a slave
1b) metaph., one who gives himself up to another's will
those whose service is used by Christ in extending and
advancing His cause among men
1c) devoted to another to the disregard of one's own interests
2) a servant, attendant