In the following piece, Martineau gives his interpretation of John 15:15, "Henceforth I call you not servants; for the servant knoweth not what his lord doeth; but I have called you friends; for all things that I have heard of my Father, I have made known to you."

I'll list here a few things he said to briefly summarize his explanation of the "servant" mind -

"Considerations of self-interest, suggestions of hope and fear, appealing to that desire of happiness or rather that recoil from suffering.  The tribute of weakness to superior power, a sacrifice extorted by necessity, an acquiescence indifferently given to the decrees of an iron Fate or the laws of the divinest Providence.

 Now he describes serving Christ as "friends" in what I think is the most insightful explanation I've ever read.  

"The Son of Man does not speak to us as strangers to a voice like his: he never moves imperiously about, as among a race of spiritual serfs, who must be made to do an outside will they are not fit to comprehend. He doubtless addresses us in the imperative voice of divine right; but not till he has made the whisper of our own conscience speak in the very same tones. He pronounces, with the calmness of inspiration, on the sublimest truths; but not without transposing us into a temper which those truths evidence themselves.

His tones are directed, not to overpower, but to penetrate. He does not bear down against resistance, but touches the springs of native force. He appeals as to souls that bear kindred with his own; that secretly know the right from which, in the misery of delusion, they have turned away; that deeply love the purity and power of heart they have so sadly lost; and feel the shame and sorrow of an alienation, boasted of perhaps as freedom, but lamented with the hidden sighs of exile. He speaks as if his diviner sphere of thought created no separation, and made no difference in the free outpouring of his soul. And so it really was: he had but to be himself and live that godlike life, to become a central light of human trust, and the most enduring object of human affection." James Martineau.