Matt. 18:1: "At that time the disciples came (προσῆλθον) to Jesus and said, 'Who then is greatest in the kingdom of heaven?'"
Matt. 18:21: "Then Peter came (προσελθών) and said to him, 'Lord, how often shall my brother sin against me and I forgive him? Up to seven times?'"By the way, the prefixed preposition (the προσ- in προσῆλθον and προσελθών) is just a common occurrence in Koine Greek. When someone goes to someone or something, you'll find πρός. When someone goes into something like a house, sometimes you'll find εἰς prefixed. And it works with other prepositions. It happens. It assigns movement or direction to the verb, you know, just a natural way of being clearer in communication. We do it in English, for example, when I tell my wife, "I'm going down to the office." Of course, English doesn't "prefix" prepositions, but that example still works. I could just say, "I'm going to the office," but "inserting "down" throws in some extra details (e.g., where I am in relationship to my office –––> higher than [altitude] or north of [geography]). Truth is, we just do this naturally. It takes almost no thought. And these prefixed prepositions in Greek took little if any at all also. It's just part of Greek, and sometimes you'll find the repetition of the prefixed preposition (or a synonym) as part of the sentence predicate. But that's not the focus of what we're trying to talk about in this post.
Now, I'm going to have to give you a little more of the passages just so you can see what's going on. The disciples mentioned in 18:1 are not present for Jesus' teaching on the temple tax (Matt. 17:24–27). But Peter is present for the teaching that follows (Matt. 18:1–20). Both indicate a person or group that comes to Jesus, but one is a group that is previously absent and the other a person who is present. The former indicates that the disciples "showed up" or arrived and the other that Peter came forward, kind of like when we say someone raises his hand to speak.
How do we make sense out of the temple tax passage? Matthew writes, "When they came to Capernaum . . ." (17:24). The "they" in that verse refers to the whole group––Jesus and his disciples. So after that, something happens. It looks like the movement goes as follows:
1. Tax-collectors approach Peter (17:24)
2. Peter came to the house (where Jesus is) (17:25)
3. Peter went to catch a fish (implied) (17:27)
4. The disciples came to Jesus (18:1)The question is what takes place between 17:27 and 18:1. The other disciples arrive at the house after Peter and Jesus have this conversation. If we could recreate the events, they could go something like this: The tax-collectors approach Peter about the temple tax; the other disciples were also present, but they directed their question to Peter (possibly another indication of his role as leader among them). Peter answers their question in the affirmative and heads off to the house (1) to ask Jesus if he answered the question correctly and (2) to get money to pay the tax. Jesus teaches Peter and then sends him back to pay the tax. Then they all return to the house, fighting amongst themselves regarding greatness in the kingdom. The problem with the view that the disciples were there with Peter when the tax-collectors arrive is Jesus did not mention enough money to cover the tax for all his cohort, only for himself and Peter (Matt. 17:27). This leads to another possibility: The tax-collectors approach Peter, who was out by himself in Capernaum; the other disciples were somewhere else in Capernaum, just not in the same place as Peter. Peter answers the question and runs to the house for the same reasons mentioned earlier. Once he leaves to retrieve the temple tax, one of two things take place. Either the other disciples show up sans Peter and are addressed by Jesus for the heated discussion they had on the way to the house regarding greatness in the kingdom, or Peter catches up with them once he leaves the house and they all fight about who the greatest is before heading back to the house together. The clause προσῆλθον οἱ μαθηταὶ τῷ Ἰησοῦ λέγοντες in Matt. 18:1 is parallel to προσελθὼν ὁ Πέτρος εἶπεν αὐτῷ in Matt. 18:21. That does not mean, however, that Peter was absent for the teachings prior to v. 21. The provision in 17:27 is really what rules out the disciples’ presence for the teaching on the temple tax.