Is Matthew 17:21 a later addition?

Why were the disciples not able to drive the demon out of the son? Did Jesus give one reason, or two?
According to older Versions like the King James Version, two reasons were given: “And Jesus said unto them, 1) Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. 2) Howbeit this kind goeth not out but by prayer and fasting.”
Modern Versions like the New International Version, mention only the first reason: “He replied, ’Because you have so little faith. I tell you the truth, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there’ and it will move. Nothing will be impossible for you.’”

Let us consider the available Manuscripts:

Matthew 17:21

External criteria.
What can we learn from the available manuscripts? Both the codices Sinaiticus and Vaticanus from around 350 A.D. do not contain verse 21. This passage is found in Codex Washingtonianus, dated about 50 years later. This codex is curiously compiled of fragments of different textual tradition, probably going back to an ancestor pasted together after Deocletian tried to destroy Christianity by burning all their sacred books he could find. In Matthew it is typical Byzantine. Most of the later manuscripts do contain this passage.
Looking at antique Translations, this passage is lacking in translations spread from Egypt through Ethiopia and Syria, Georgia between the Black and Caspian Seas, and even into Europe in Old Latin translations. Yet it is found mainly in old Latin translations and the Vulgate, but also in Syria, Russia and Ethiopia.
How much weight should we give to the oldest and most widely spread translations?

Internal criteria.
What would a scribe normally do when he was confronted by two differing versions in his source and controlling manuscripts? To ensure that he would not leave out some passage that might be part of the original, he would rather include than leave out. That is why in general manuscripts of later dates tend to be longer
A document that immediately calls for our attention concerning this verse, is the Diatessaron (±170 A.D.). Tatian, the Syrian compiled a single narrative of the four Gospels. It was quite popular at the beginning since one document instead of four Gospels would contain all. Due to its wide circulation, it could easily serve as control document for correcting future copies, but could just as easy result in including segments from other gospels, like this presumed interpolation from Mark 9:29.

Intrinsic criteria:
In verse 20 Jesus gives an elaborate explanation of why the lack of faith on the side of the disciples could hinder the casting out of the demon. Mark mentions the importance of prayer. If indeed this passage had been interpolated from Mark, it actually stands on its own, like an afterthought. The authenticity of a report is not confirmed by congruency with other reports, but rather by the uniqueness and function of each part to the whole.

The effect of the addition of this passage to Matthew:
The healing of this son is reported in three gospels, with great correspondence of the main detail on the healing itself. Luke (37-43) does not discuss the question of why the disciples failed to drive out the demon. Mark blames the disciples’ inability on lack of prayer. In Matthew we find the lack of faith. Yet this peculiar description of faith likening it to the mustard seed, is also found among a few non related sayings of Jesus in Luke 17:6.
That each Gospel mentions what its source remembers, and that it might differ from the detail of another, points to authenticity rather than harmonizing the facts.

God bless,

Herman.

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