Actually Martin the context does help you understand vs 48 in fact we have to use the context if we want to come to understand that verse.
The Greek word for “appointed” or “ordained tassō G5021used in this verse could either be in the middle or passive voice, as both are spelled the same way in Greek. So the only way to understand this verse is by looking at the context [vs 46-48] which as you know contrasts the rejection of the message by the Jews and the acceptance of it by the Gentiles.
They believed," trusted, or were saved, when they believed, after the word was preached, Mark 1:15; Acts of the Apostles 16:31; Romans 1:15-16
It affirms that only those that were convicted by the Holy Spirit that accompanied the apostles' preaching were convicted and believed unto eternal life, 1 John 5:13.
Revelation 3:7 is clear that no one can force their way in to heaven but that is not what Acts 13:46-48 is showing us is it. Those of the gentiles that believed were saved by grace because of their faith. Why do you deny what the context clearly shows us.[/QUOTE]
Context is always important, but it doesn't help you here. The Greek word is tetagmenoi, which is the Perfect Passive Participle of tasso. I don't believe there is any usage of tetagmenoi which is clearly Middle. But the Perfect tense means a once for all completed event in the past. 'As many believed, having been appointed for etrrnal life.' So the appointment comes before the believing. So how would you translate it using the Middle Voice? So as many believed, having appointed themselves to eternal life'? Is that what you believe?
It is clear that no one is saved who doesn't trust in Christ. It is equally clear that no one believes wthout God's election. Both these things have to be held together. There is a very well-known saying which goes back, I think, to Spurgeon, which relates that there is a sign above the door which leads to the kingdom of heaven, saying, Whosoever will may enter.' But having entered, when you look back, you will read on the other side of the door, 'Chosen from before the foundation of the world.' Verse 48 stresses the Divine agency, and that is why 'The disciples were filled with joy and with the Holy Spirit' (verse 52; cf. Luke 10:21)..
BTW, half of Revelation 3:7 says that no one can force his way into heaven, but the other half says that the Triune God, and He alone, brings people in there.