“The Peter answered and said to Jesus, ‘Rabbi, it is good for us to be here; and let us make three tabernacles: one of You, one for Moses, and one for Elijah’
“because he did not know what to say, for they were greatly afraid” (Mark 9:5-6).
Tense, stressful situations and even fear can cause people to react in unusual ways that, in other circumstances, they would handle in a much more calculated way. That seems to be the situation with the Apostle Peter here.
In fact, Peter here demonstrates the kind of impulsiveness that he has also shown in other situations in scripture. For some reason, he feels the need to say something, and he just does not know what to say. Miraculously seeing Moses and Elijah, Peter concludes they are on a parallel with Jesus, who is with them; thus, he hastily and incorrectly spurts out this suggestion.
One lesson a person can take away from this situation is that, if one does not know what to say, he would be better off to think first and then speak or even not say anything at all. The incident does, however, provide God an ideal setting for declaring to the world (verse 7) that Jesus is the One to hear, no doubt by divine design.