Concerning the Old Man in Shakespeare's Macbeth, the enotes Study Guide on the play has this to say about the Old Man:
The anonymous old man represents experience and memory, and is at least 70 years old ("Threescore and ten I can remember well" he says in II.iv.1). He comments on the disturbances in nature on the night of Duncan's murder, unprecedented in his recollection. He is referred to by Rosse several times as father. He wishes a blessing on Rosse as he travels to Scone.
The Old Man may be a commoner, and he does wish a blessing, but it might be a bit of a stretch to conclude that he "represents" common people and Christianity. Even if he is both common and strongly Christian, one should be careful of over symbolizing dramatic elements, including characters.
The Old Man is present in the scene for at least three purposes: to further the idea of the unnatural (the falcon attacked by an owl), to witness the conversation between Ross and Macduff about who is responsible for the murders of Duncan and the two grooms, and to give the blessing,
God's benison [blessing] go with you, and with those
That would make good of bad, and friends of foes. (Act 2.4.41-42)
thereby also furthering, in addition to the "unnatural" theme mentioned previously, the theme of duality and opposition.
The old man plays the role of commentator. He is a peripheral character and is not directly involved in the plot and, thus, cannot affect the sequence of events. He is, however, important because he provides an objective point of view about events. His comments create further awareness of the supernatural occurrences accompanying the unnatural actions of the lead characters, Macbeth and his lady.
The old man emphasizes the "perturbation in nature" which is a common element throughout the play. He states, for example:
Threescore and ten I can remember well:
Within the volume of which time I have seen
Hours dreadful and things strange; but this sore night
Hath trifled former knowings.
The reference to his old age and the fact that he has never, in his entire life up to now, witnessed such strange events further accentuates the point. He mentions that the strange occurrences that he has witnessed during this particular night by far exceed anything that he has ever seen before.
The old man makes comparisons between Duncan's ghastly murder and events in nature that he has witnessed. He states that both were irregular. The point is that a king should die a natural death and that succession to his throne should also be natural. In saying what he does, he also foreshadows a further unnatural occasion: Macbeth's usurpation of the throne. It is Malcolm, who has been named Prince of Cumberland and, therefore, Duncan's successor, who should occupy this position, but he has fled the kingdom with his brother.
The old man is also witness to the charged conversation between Macduff and Ross. It is pertinently clear from their discussion that Macduff does not feel any loyalty to Macbeth. He is not prepared to attend his coronation and would rather go to his castle at Fife. This is, obviously, a deliberate slap in Macbeth's face and an indication of Macduff's mistrust. This is further highlighted by his deeply ironic comment:
Well, may you see things well done there: adieu!
Lest our old robes sit easier than our new!
The old man's good-natured farewell is also an indication of the general hope that all will go well with Scotland and its people. He extends God's blessings to the two departing generals and, in a dramatic and verbally ironic expression of another theme (the duality of things), he extends a blessing even to those who wish to turn good into bad, as we know Macbeth and his wife have and will continue to do.