So are you saying that when someone accepts God, then they do not commit sins after that time and thus there is no need to forgive these sins? People commit sins before and after they come to Christ. The difference is, according to 1 John is that people do not "practice" sinning.
1Jn 3:9-10 No one born of God makes a practice of sinning, for God's seed abides in him, and he cannot keep on sinning because he has been born of God. By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.
We are either children of God and practice righteousness or we are children of the devil who practice sin.
...Please explain the following: “If God has already forgiven all of a man’s sins, or will forgive them all (past and future) upon a single act of repentance, then it makes little sense to tell the apostles they have been given the power to “retain” sins, since forgiveness would be all-or-nothing and nothing could be “retained.”
You are making an assumption that the message in John was only to the apostles. If you read Luke 24:33 you will find it states:
Luk 24:33 And they rose that same hour and returned to Jerusalem. And they found the eleven and those who were with them gathered together,
There were more than apostles gathered in that upper room.
Scripture also teaches that only God can forgive sin (Mark 2:7)
So what did Christ mean when He stated:
Joh 20:23 If you forgive the sins of any, they are forgiven them; if you withhold forgiveness from any, it is withheld."
The way Catholics would interpret this would mean that 1) if some are going to hell then Catholics are withholding forgiveness from some, or 2) no one is going to hell because no one is withholding forgiveness. Each of these lines of reasoning is simply wrong.
A more simplier and harmounious interpretation is that Christ is saying if Christians declare that those who genuinely repent and believe the gospel, they will have their sins forgiven by God. Those who don't repent will die in their sins and they will go to hell.
It is no different then the following:
Joh 9:34 They answered him, "You were born in utter sin, and would you teach us?" And they cast him out.
Joh 9:35 Jesus heard that they had cast him out, and having found him he said, "Do you believe in the Son of Man?"
Joh 9:36 He answered, "And who is he, sir, that I may believe in him?"
Joh 9:37 Jesus said to him, "You have seen him, and it is he who is speaking to you."
Joh 9:38 He said, "Lord, I believe," and he worshiped him. Joh 9:39 Jesus said, "For judgment I came into this world, that those who do not see may see, and those who see may become blind."
Joh 9:40 Some of the Pharisees near him heard these things, and said to him, "Are we also blind?"
Joh 9:41 Jesus said to them, "If you were blind, you would have no guilt; but now that you say, 'We see,' your guilt remains.
Our Lord extended forgiveness. Our Lord withheld forgivenss.