James in theEvangelical Exegetical Commentary (underlining mine):
The social and conceptual stock of δουλος has attracted much attention in recent years. At its most basic level, the word denotes literal ownership by another. There is an abundant amount of linguistic evidence in nonbiblical literature stretching from early writers through the first century A.D. that a δουλος was a person who was owned by another free person (Plato, Gorg. 57 §502D; Dio Chrysostom 9 , 4; Corp. herm. 18, 5; Philo, Sacr. 26; Abr. 251; Good 136, 139; Josephus, Ant. 16.126; War 7.336). While we cannot be entirely certain, some have estimated that 20 percent of the people living in the Roman Empire were slaves in the first century.
We commit anachronism if we conjure up images of the modern African slave trade with the slave culture that was associated with δουλος as we read the language in James 1:1. While it is true that many could earn their freedom, a privilege not usually associated with African slavery, the key characteristic of a δουλος was his or her lack of freedom. This can also be seen in the word being set over against its opposite, ελεύθερος, as in 1 Corinthians 12:13: εἴτε δουλοι εἴτε ελεύθεροι (see also Gal 3:28; 4:22–31; Eph 6:8; Col 3:11; Rev 6:15; 13:16; 19:18). It is highly questionable, therefore, to translate δουλος as “servant,” because it may tend toward an anachronistic obscuring of its meaning both in the secular and the sacred literature of the ancient world. Thus, in biblical terms the Israelites were very much “slaves (not “servants”) in the land of Egypt in the house of Pharaoh” (LXX 1 Sam 2:27). They could not buy their way out from this slavery.
The concept of ownership by another with its consequent lack of rights is abundantly clear in the linguistic usage of the δουλος in the Greco-Roman world of the NT. But have we exhausted its meaning when we only view the word against its Greek background? When we consider the word in its Jewish cultural context, another idea emerges. Religiously, δουλος connotes a special relationship between God and humans defined in terms of possession (by God) and service (by humans). The perception of Paul and his companions was that they were “slaves of the Most High God” (Acts 16:17). In the Hebrew Bible, the term עבד is used to define such a religious relationship. Israel is called “slave of the Lord” in the LXX of Psalms 134:1; 135:22 [136.22]; Isa 49:3; and Ezekiel 28:25. The religious expression of slavery as dedication to God permeates the piety of the Psalms (LXX Pss 118:38, 76; 122:2; 133:1; 135:1; 142:12). In Isa 42:19, the Hebrew “servant of the Lord” is rendered by the LXX in the plural as δουλοι του θεου (“slaves of God”), the only instance of this phrase in the LXX. The term δουλος is often also attributed to those leaders who mediate between God and humans, such as Joshua (Josh 14:7; 24:30; Judg 2:8), David (2 Sam 7:8, 25, 29; 1 Chr 17:4), and Moses (LXX Ps 104:26, 42; Mal 3:24). More often, it is used of the prophets as messengers of the Lord (Amos 3:7; Joel 3:2; Jonah 1:9; Zech 1:6; Jer 7:25; 25:4; Ezek 38:17).
In the NT likewise, the term can be applied to Jesus (Phil 2:7) or to Christians generally (1 Pet 2:16; Acts 2:18; 4:29; Rev 10:7; 19:5; 22:3, 6). My important point, however, is that it also appears as a title for Christian leaders, either in the form “slave of Jesus Christ” (Rom 1:1; Phil 1:1; 2 Pet 1:1) or “slave of Christ” (Gal 1:10). Only in Titus 1:1 does the specific title δουλος θεου refer to Paul. In light of this usage, especially in the OT, we must ask whether the functional sense of the word should be seen against a Hellenistic-Roman background (“slave” in the basest sense) or against its Jewish background (a “servant” like the prophets and other Israelite leaders). The thorough Jewishness of James and his readers should point us toward the latter context (Vincent, 116; Dibelius, 66; Martin, 7; Adamson, 49–51; Johnson, 171). Thus the term stresses more the noble role of a prophet/apostle/servant than that of an ignominious slave with no rights. Actually it is best to understand the self-title as both (1) an indication of humility, for the servant does not come in his own name, and (2) a description of an office, for the bearer of this title is in the service of a great king (Mussner, 60–61).