“Who really is the faithful and discreet* slave whom his master appointed over his domestics, to give them their food at the proper time? - Matthew 24:45
Let's be honest, the faithful and discreet slave logically cannot refer to any and every Christian. This is not merely an encouragement for all Christians to prove themselves faithful and discreet. The text says that the faithful and discreet slave is "appointed over" Jesus' "domestics" or his "body of attendants" (Luke 12:42). Thus the faithful and discreet slave obviously refers to one(s) in a position of responsibility or authority over others. Who? Well, who are the "domestics"?
Luke refers to them as being Jesus' "body of attendants". Several scriptures refer to the congregation as being Christ's body. I think it's very reasonable to conclude that the domestics or body of attendants refers to the congregation. Therefore the faithful and discreet slave refers to one(s) having authority over the congregation. Authority for what purpose?
Jesus says their role would be to feed the domestics. Does this not remind you of Jesus' words to Peter: "Feed my little sheep"? Jesus appointed Peter to feed the newly formed Christian congregation. Thus it can be clearly seen that Peter falls within the role of the faithful and discreet slave mentioned at Matthew 24:45. Does it end there? No.
Jesus' direction to Peter: "feed my little sheep" implicitly refers to Peter as being a shepherd. For shepherds feed sheep by leading them to their pastures of food. Who else does the NT refer to as being shepherds, thus showing they have a role of feeding the congregation? Elders. A chief role of an elder is teaching - providing spiritual food. Elders are appointed over the congregation. They are "overseers". Thus elders also fall within the role of the faithful and discreet slave mentioned at Matthew 24:45.
So my conclusion is that the faithful and discreet slave does not refer to a specific group of men serving on a Governing Body at a particular time in history. No. It refers to all Christians living at all times who are appointed to serve as teachers of the congregation. It applies to Peter and the apostles and it applies to elders. It applies to all who are appointed with authority to teach the congregation. So the parable of Matthew 24:45-51 and Luke 12:42-46 is an encouragment/warning to all who would be appointed to teach the congregation, to prove themselves faithful and discreet.
Further corroborating this point is the statement in the parable, that the slave will be punished with "the greatest severity" if found unfaithful when the master returns. This matches the scripture in James that say: "Not many of you should become teachers as you would face heavier judgement." We also see that the elders in the congregation - the stars in Jesus' hand - are the ones that Jesus addresses when he counsels the congregations in the opening chapters of the book of Revelation. This harmonizes with the idea that they are the slaves appointed to feed the domestics and have to answer for the state of the congregation. For further evidence, have a read of the scriptures that give the qualifications for elders and contrast it with what Jesus says about the evil slave in the FDS parable.