The NAS New Testament Greek Lexicon
Doulos "a slave, bondman, man of servile condition"
Psalm 116:16 (KJV)
O LORD, truly I am Thy servant. Thou has loosed my bonds [you have freed me from my chains].
Samuel Lavington in Spurgeon's The Treasury of David from Sermons, and Other Discourses
https://archive.org/details/sermonsandother01lavigoog
Thou hast made me free...Thou hast broken the bonds of sin...Thou hast delivered me from the tyranny of Satan...make me as one of Thy servants. I owe my liberty, my life, and all that I have, or hope, to Thy rescue; and now, my gracious Friend and Redeemer, I lay myself and my all at Thy feet.
Matthew 20:26-27
Whoever wants to become great among you must be your servant (diakanos), and whoever wants to be first must be your slave (doulos)...
Romans 6:16 & 22, 14:8
Don’t you know that when you offer yourselves to someone as obedient slaves, you are slaves of the one you obey—whether you are slaves to sin, which leads to death, or to obedience, which leads to righteousness? Now you have been set free from sin and have become slaves of God… If we live, we live for the Lord; and if we die, we die for the Lord. So, whether we live or die, we belong to the Lord.
1 Corinthians 7:22b-23a
He who was a free man when he was called is Christ's slave. You were bought at a price...
Revelation 5:9b
With your blood you purchased men for God...
Richard Baxter A Christian Directory in Voices From The Past - Puritan Devotional Readings
If you have sincerely given yourself up to God, and consented to his covenant, show it, by turning the face of your endeavours and conversation quite another way; seek heaven more fervently and diligently than you ever sought the world or fleshly pleasures. Holiness consists not in a mere forbearance of a sensual life, but principally in living unto God. The principles or heart of holiness is within, and consists in the love of God, his Word, his ways, his servants, his hounour, and his interest in the world. It consists in the soul's delight in God, and the ways of God. It is inclined towards him, and seeks after him to please him. It hates to offend him. The expression of it in our lives consists in a constant, diligent exercise of the internal life, according to the directions of the Word of God.
If you are a believer, and have subjected yourself to God as your absolute Sovereign, King, and Judge, it will then be your work to obey and please him, as a child his father, or a servant his master. God calls you not only to do no harm, but to love and serve him with all your heart, and soul, and might. If you have a better master than you had before, you should do more work than you did before. Will you not serve God more zealously than you served the devil? Will you not labour harder to save your souls than you did to damn them? Will you not be more zealous in good than you were in evil? If you are true believers, you have now laid up your hope in heaven; seek it, as worldlings set themselves to seek the world.
F.B. Meyer
I shall never forget when I first entered into the realization of the Ownership of my Lord; that I was His chattel, and had no longer any option or choice for one's enjoyment or emolument. The life which was commenced then has been one of perfect freedom, for this is the enigma of His service, that Christ's slaves are alone free; and that the more absolutely they obey Him, the more completely do they drink of the sweet cup of liberty!
R.C. Sproul
The only freedom that man ever has is when he becomes a slave to Jesus Christ.
A.B. Simpson
The doulos spirit is the spirit of self-renunciation and glad submission to proper authority-service utterly disinterested, yielding its own preferences and interests unreservedly for the glory of the Master and the sake of our brethren. Lord, clothe us with such humility and make us wholly Thine.
John MacArthur
When you give somebody the gospel, you are saying to them, “I would like to invite you to become a slave of Jesus Christ. I would like to invite you to give up your independence, give up your freedom, submit yourself to another's will, abandon all your rights, be owned by, controlled by the Lord.”