doúlos – properly, someone who belongs to another; a bond-slave, without any ownership rights of their own. Ironically, 1401 /doúlos (“bond-slave”) is used with the highest dignity in the NT – namely, of believers who willingly live under Christ’s authority as His devoted followers.
Ephesians 6:5-9 NKJV
Bondservants, be obedient to those who are your masters according to the flesh, with fear and trembling, in sincerity of heart, as to Christ;  not with eyeservice, as men-pleasers, but as bondservants of Christ, doing the will of God from the heart,  with goodwill doing service, as to the Lord, and not to men,  knowing that whatever good anyone does, he will receive the same from the Lord, whether he is a slave or free.  And you, masters, do the same things to them, giving up threatening, knowing that your own Master also is in heaven, and there is no partiality with Him.
Colossians 3:22-25 NKJV
Bondservants, obey in all things your masters according to the flesh, not with eyeservice, as men-pleasers, but in sincerity of heart, fearing God.  And whatever you do, do it heartily, as to the Lord and not to men,  knowing that from the Lord you will receive the reward of the inheritance; for you serve the Lord Christ.  But he who does wrong will be repaid for what he has done, and there is no partiality.
1 Timothy 6:1-2 NKJV
Let as many bondservants as are under the yoke count their own masters worthy of all honor, so that the name of God and His doctrine may not be blasphemed.  And those who have believing masters, let them not despise them because they are brethren, but rather serve them because those who are benefited are believers and beloved. Teach and exhort these things.
Romans 1:1-4 NKJV
Paul, a bondservant of Jesus Christ, called to be an apostle, separated to the gospel of God  which He promised before through His prophets in the Holy Scriptures,  concerning His Son Jesus Christ our Lord, who was born of the seed of David according to the flesh,  and declared to be the Son of God with power according to the Spirit of holiness, by the resurrection from the dead.
Titus 2:9-10 NKJV
Exhort bondservants to be obedient to their own masters, to be well pleasing in all things, not answering back,  not pilfering, but showing all good fidelity, that they may adorn the doctrine of God our Savior in all things.
1 Peter 2:18-21 NKJV
Servants, be submissive to your masters with all fear, not only to the good and gentle, but also to the harsh.  For this is commendable, if because of conscience toward God one endures grief, suffering wrongfully.  For what credit is it if, when you are beaten for your faults, you take it patiently? But when you do good and suffer, if you take it patiently, this is commendable before God.  For to this you were called, because Christ also suffered for us, leaving us an example, that you should follow His steps: