I heard John Stott teaching on this and he said that on one occasion the word is used to refer to ALL believers - a few other times it refers to those who were sent, even Jesus. I'd trust his knowledge of theology and Greek any day.
Yes I agree, you can't go wrong with John Stott. This is what John Stott says about apostles.
John Stott - The Message of Acts
b. The foundation ministry of the apostles
We have already noted that the ascension was the watershed between the two phases earthly and heavenly of the ministry of Jesus Christ. Now we need to note that he was not taken up to heaven, until after he had given instructions through the Holy Spirit to the apostles he had chosen. This is clearly emphasized in the Greek sentence, which reads literally: 'until the day when, having instructed his chosen apostles through the Holy Spirit, he was taken up.' Thus, before ending his personal ministry on earth, Jesus deliberately made provision for its continuance, still on earth (through the apostles) but from heaven (through the Holy Spirit). Because the apostles occupied a unique position, they also received a unique equipment. Luke outlines four stages:
(i) Jesus chose them
They were the apostles he had chosen (2). Luke has used the same verb ekle gomai in his account of Jesus' calling and choice of the Twelve, 'whom he also designated apostles', and he is about to use it again when two men are proposed to fill the vacancy left by Judas and the believers pray 'Lord, ... show us which of these two you have chosen. Significantly, the same verb is also used later in connection with Paul. The risen Lord describes him to Ananias as 'my chosen instrument to carry my name before the Gentiles ...' (9:15), and Ananias conveys this message to Paul: "The God of our fathers has chosen you ... You will be his witness ...' (22:14-15). It is thus emphasized that all the apostles (the Twelve, Matthias and Paul) were neither self-appointed, nor appointed by any human being, committee, synod or church, but were directly and personally chosen and appointed by Jesus Christ himself.
(ii) Jesus showed himself to them
The other evangelists have indicated that Jesus appointed the Twelve that they might be with him and so be uniquely qualified to bear witness to him. 43 The foundation witnesses had to be eyewitnesses. Judas' successor, Peter said, had to be someone who had been with the Twelve the whole time the Lord Jesus went in and out among us, beginning from John's baptism to the time when Jesus was taken up from us' (1:21-22). And in particular he must be a witness with us of his resurrection (1:22, cf. 10:41). So, after his suffering, the risen Lord showed himself to these men (3). Luke stresses this. Jesus gave them many convincing proofs (tekmērion is a 'convincing, decisive proof BAGD) that he was alive, which continued over a period of forty days. During this time he appeared to them becoming visible), spoke about the kingdom of God (so that they heard as well as saw him) and on one occasion at least was eating with them, which indicates that he was no ghost, but could be touched (10:41). He thus presented himself to their senses: their eyes, ears and hands.
Such an objective experience of the risen Lord was an indispensable qualification of an apostle, which explains why Paul could be one and James and why there have been no comparable apostles since and can be none today.
(iii) Jesus commanded or commissioned them
In addition to speaking to them about the kingdom of God (3) and the Holy Spirit (4-5), which we shall consider further in the next chapter, he gave them certain instructions through the Holy Spirit (who inspired all his teaching. What were these instructions? It is interesting that the Bezan or Western text answers this question by adding the apostles whom he had chosen and commanded to preach the gospel'. If this is correct, then the risen Lord's instruction was none other than his great commission, which Luke has already recorded at the end of his gospel in terms of preaching repentance and forgiveness in his name to all nations. Do and which Jesus will soon repeat in terms of being his witnesses to the ends of the earth (1:8). This, then, adds a further feature to the portrait of an apostle. Apostolos was an envoy, delegate or ambassador, sent out with a message and carrying the authority of the sender. Thus Jesus chose his apostles, and showed himself to them after the resurrection, as preliminaries to sending them out to preach and teach in his name
(iv) Jesus promised them the Holy Spirit
In the Upper Room, according to John, Jesus had already promised the apostles that the Spirit of truth would both remind them of what he had taught them 21 and supplement it with what he had not been able to teach them. 22 Now Jesus commands them to wait in Jerusalem until the promised gift has been received (4). It was his Father's promise (4a, presumably through such Old Testament prophecies as Joel 2:28ff., Is. 32:15 and Ezk. 36:27), his own (since Jesus had himself repeated it during his ministry, 4b), and John the Baptist's, who had called the 'gift' or 'promise' a 'baptism' (5). Jesus now echoes John's words and adds that the thrice-repeated promise (“the promised Holy Spirit', 2:33) is to be fulfilled in a few days. So they must wait. Not till God has fulfilled his promise and they have been 'clothed with power from on high', can they fulfil their commission.
Here, then, was the fourfold equipment of the apostles of Christ. Of course in a secondary sense all the disciples of Jesus can claim that he has chosen us, revealed himself to us, commissioned us as his witnesses, and both promised and given us his Spirit. Nevertheless, it is not to these general privileges that Luke is referring here, but to the special qualifications of an apostle—a personal appointment as an apostle by Jesus, an eyewitness experience of the historical Jesus, an authorizing and commissioning by Jesus to speak in his name, and the empowering Spirit of Jesus to inspire their teaching. It was primarily these uniquely qualified men through whom Jesus continued to do and to teach', and to whom Luke intends to introduce us in the Acts.
