The Distinguishing Mark of Christianity!

Jesus is Lord (1 Corinthians 12:3) is the distinguishing article of Christianity and marks the essential confession of faith (Romans 10:9). Jesus proclaimed it to His disciples, His enemies, and His casual inquirers alike — and He refused to tone down its implications.
The expression "Lord" (kurios) speaks of ownership, while "Master/Lord" (despotes) denotes an unquestionable right to command (John 13:13; Jude 4). Both words describe a master with absolute dominion over someone else. That explains Jesus' incredulity at the practice of those who paid homage to Him with their lips but not with their lives: "Why do you call Me, 'Lord, Lord,' and do not do what I say?" (Luke 6:46).
Doulos frequently describes what it means to be a true Christian: "He who was called while free, is Christ's slave [doulos]. You were bought with a price" (1 Corinthians 7:22-23). It describes the lowest, abject bond slave; his service is not a matter of choice.

A Misleading Translation
Unfortunately, readers of the English Bible have long been shielded from the full force of doulos because of an ages-old tendency to translate it as "servant" or "bond-servant." This tendency is regrettable, since service and slavery are not the same thing. "No one can be a slave to two masters" (Matthew 6:24) makes better sense than "No one can serve two masters." An employee with two jobs could indeed serve two masters; but a slave could not. Scripture repeatedly calls Christians "slaves" (1 Corinthians 6:19-20), purchased for God (Revelation 5:9). This is the very essence of what it means to be a Christian (Romans 14:7-9).

A Revolting Concept
Not only is slave a word loaded with negative connotations, but our generation is also fixated on the concepts of freedom, fulfillment, and autonomy. Saving faith and Christian discipleship have been reduced to the cliché "a personal relationship with Jesus." It's hard to imagine a more disastrous twisting of what it means to be a Christian. Many people (including Judas and Satan) had some kind of "personal relationship" with Jesus during His earthly ministry without submitting to Him as Lord. But His only true friends were those who did what He said (John 15:14).

A Difficult Truth
Slavery to Christ is not a minor or secondary feature of true discipleship. It is exactly how Jesus Himself defined the "personal relationship" He must have with every true follower (John 12:26; 15:20). In fact, the fundamental aspects of slavery are the very features of redemption. We are chosen (Ephesians 1:4-5; 1 Peter 1:2; 2:9); bought (1 Corinthians 6:20; 7:23); owned (Romans 14:7-9; 1 Corinthians 6:19); subject to His will and control (Acts 5:29; Philippians 2:5-8); called to account (Romans 14:12); evaluated (2 Corinthians 5:10); and either chastened or rewarded by Him (1 Corinthians 3:14; Hebrews 12:5-11). Those are all essential components of slavery.

A Divine Introduction
Jesus introduced the NT slave metaphor. He frequently drew a direct connection between slavery and discipleship (Matthew 10:24-25). His words reflect what every true disciple should hope to hear at the end of life: "Well done, good and faithful slave. You were faithful with a few things, I will put you in charge of many things; enter into the joy of your master" (Matthew 25:21).
Jesus always described true discipleship in such terms, with no effort to adjust the message to make it sound appealing to worldly-minded sinners. He never muted what it would cost to follow Him. Would-be disciples who tried to dictate different terms were always turned away (Luke 9:59-62).

Slaves Who Are Friends
Perhaps the key passage on Jesus' demand for implicit obedience is one already alluded to — John 15:14-15: "You are My friends if you do what I command you. No longer do I call you slaves, for the slave does not know what his master is doing; but I have called you friends, for all things that I have heard from My Father I have made known to you."
The fundamental principle here is obedience. Jesus was not suggesting that His favor could be earned through service. Rather, He was saying that obedience is a singular proof that someone is His friend. Implicit obedience to His commandments is the natural fruit of genuine love for Him - the telltale mark of authentic, saving faith.
Why, then, does He say, "No longer do I call you slaves…I have called you friends" (v. 15)? Is He expressly telling them their relationship with Him was now a familiar, personal camaraderie between colleagues, rather than a master-slave relationship governed by authority and submission?
Not at all. The apostles were still His slaves, because that's precisely what they were. He was simply saying they were His friends as well as His slaves. "The slave," He explains, "does not know what his master is doing." A slave isn't owed any explanation or rationale. But Jesus had kept nothing secret from His disciples: "all things that I have heard from My Father I have made known to you" (v. 15). They were therefore much more than mere slaves to Him. They were His friends as well, privy to His thoughts and purposes (cf. 1 Corinthians 2:16).

Slavery and True Liberty
So understood correctly, the gospel is an invitation to slavery. On the one hand, the gospel is a proclamation of freedom to sin's captives and liberty to people who are broken by the bondage of sin's power over them. On the other hand, it is a summons to a whole different kind of slavery: "Having been freed from sin, you became slaves of righteousness" (Romans 6:18; cf. 1 Peter 2:16).
Both sides of the equation are vital. There is a glorious freedom in being the slaves of Christ (John 8:36), but it means the end of human autonomy for the true follower of Christ. In other words, everyone serves some master. We are all enslaved in one way or the other (Romans 6:16-21).
There is no legitimate way to adjust the message in order to make it sound appealing to people who admire Jesus but aren't prepared to serve Him. Jesus didn't seek admirers; He was calling followers — not casual followers, but slaves. Remove that spirit of submission, and the most profound kind of "admiration" for Christ is a spiritual fraud that has nothing to do with true faith.