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Do we call men "master" or not?
Matt. 23:10 "Neither be called masters, for you have one master, the Christ..."
Eph. 6:5 "Servants, be obedient to them that are your masters according to the unto Christ."

So do we call men "master" or don't we?

To start, there's a difference behind the Greek. Paul uses kurios, an all-purpose word equal to "sir" in modern terms. Jesus uses kathegetes meaning guide or teacher. This matches the context of the Matthew, which warns against using titles associated with teaching (Rabbi, Father) and is in the context of warnings against those who teach the law.

That raises another issue: Do we apply this to calling our biological fathers "father"? Not at all, since this too was a title used by the ancient rabbis.

Here is a related issue:

1 Peter 2:18 Servants, be subject to your masters with all fear; not only to the good and gentle, but also to the forward.

How does this square with Jesus' words to Satan, to serve God only? It is matched up in part by the precept that God sets earthly authorities to be obeyed (Romans 7), and in that sense, we serve God through masters. However, Peter specifies that we obey masters; the word used by Jesus, latreuo means "render religious homage," or worship -- not the same thing.

Objection: Kathegetes does not necessarily mean teacher or guide because the chapter could equally well be said to be in the context of warnings against obeying masters in the master/servant sense. See verse 4.

For they (the scribes and Pharisees) bind heavy burdens and grievous to be borne, and lay them on men's shoulders; but they themselves will not move them with one of their fingers.

The application is in error. The scribes and Pharisees bound the people with heavy burdens through their teachings rather than by literally being in command of the rank and file Jews. Note as well that kathegetes appears nowhere else in the NT but in the passage in Matthew.

Indeed, rank and file Jews would have given the scribes and Pharisees little attention. They went around harrassing people, but in terms of "enforcement" there wasn't a whole lot they could do about it. There just wasn't much call for jailing people for picking grain on the Sabbath, and the Romans kept the Sanhedrin from enforcing more serious penalties without permission.

Kurios, which is used by Colossians and Ephesians to describe whom servants should obey, is a term of respect that means, among other things, "God". Isn't this a far more serious matter than what Jesus talks about in Matthew 23? If we are not even to call men "teacher" or "father" because those titles are reserved to God - how much worse it must be to call one's fellow mortals by a title that does in fact mean God?

Kurios does not "mean" God. It means "lord" or "sir" and as such was an appropriate title for God (or a god) in the ancient world, or for any person one respected. If it meant "God" then the Greeks called Philip "God" in John 12:21.