In the Bible you could become a slave and be set free in a jubile year. The only problem with this is people started to ignore God's command and kept the slaves beyond jubile.
Leviticus 25:40 *But as an hired servant, and as a sojourner, he shall be with thee, and shall serve thee unto the year of jubile:
Am I wrong in thinking Paul asked Philemon to free Onesimus but didn't demand it by saying he's not a Cristian if he doesn't free his slaves?
Philemon 1
1 ¶Paul, a prisoner of Jesus Christ, and Timothy our brother, unto Philemon our dearly beloved, and fellowlabourer,
2 And to our beloved Apphia, and Archippus our fellowsoldier, and to the church in thy house:
3 Grace to you, and peace, from God our Father and the Lord Jesus Christ.
4 I thank my God, making mention of thee always in my prayers,
5 Hearing of thy love and faith, which thou hast toward the Lord Jesus, and toward all saints;
6 That the communication of thy faith may become effectual by the acknowledging of every good thing which is in you in Christ Jesus.
7 For we have great joy and consolation in thy love, because the bowels of the saints are refreshed by thee, brother.
8 ¶Wherefore, though I might be much bold in Christ to enjoin thee that which is convenient,
9 Yet for love's sake I rather beseech thee, being such an one as Paul the aged, and now also a prisoner of Jesus Christ.
10 I beseech thee for my son Onesimus, whom I have begotten in my bonds:
11 Which in time past was to thee unprofitable, but now profitable to thee and to me:
12 Whom I have sent again: thou therefore receive him, that is, mine own bowels:
13 Whom I would have retained with me, that in thy stead he might have ministered unto me in the bonds of the gospel:
14 But without thy mind would I do nothing; that thy benefit should not be as it were of necessity, but willingly.
15 For perhaps he therefore departed for a season, that thou shouldest receive him for ever;
16 Not now as a servant, but above a servant, a brother beloved, specially to me, but how much more unto thee, both in the flesh, and in the Lord?
Then the verse where you could become a slave for life by your own choice if you love a slave woman and get a hole poked in your ear.
Deuteronomy 15:15 *And thou shalt remember that thou wast a bondman in the land of Egypt, and the LORD thy God redeemed thee: therefore I command thee this thing to day.
16 *And it shall be, if he say unto thee, I will not go away from thee; because he loveth thee and thine house, because he is well with thee;
17 *Then thou shalt take an aul, and thrust it through his ear unto the door, and he shall be thy servant for ever. And also unto thy maidservant thou shalt do likewise.