Are our sins of the future forgiven? (John 20:23) Catholics say that our future sins are not forgiven (unless we confess them in the future) and say that John 20:23 (and maybe other verses which I can't remember), uses the past, i.e. "forgiven", so ask how future sins can be forgiven. 

Is this interpretation justified?  What information there is on the verb "forgiven" here, and in other verses, that may support or contradict this interpretation?
The OP asks:


  Catholics say that our future sins are not forgiven...and say that John 20:23...uses the past, i.e. "forgiven", so ask how future sins can be forgiven. What information there is on the verb "forgiven” here...? 


Since nobody seems to have addressed the issue of verb tense here, I’ll give it a try. The verb in question is infected as a third person plural, perfect passive indicative.1 


  ἄν τινων ἀφῆτε τὰς ἁμαρτίας ἀφέωνται αὐτοῖς...
  If you (pl.) forgive the sins of any (pl), they are forgiven them...


The identification of this verb as "past" in the question presumably derives from the perfect tense. However, aspect rather than time is the primary value of the Greek tenses. People argue2 about the extent to which Greek verbs contain time information at all. 

Regardless of your stance on Greek tenses, the understanding of ἀφέωνται here is heavily tied up with the fact that it forms the apodosis of a conditional statement.3  The form &ndash; (ε)αν + subjunctive + any mood/tense &ndash;  is known as a third class conditional. Note in that formula the lack of specification of the mood or tense of the apodotic verb. For the purpose of framing the contingency, that is largely irrelevant.3 

This is not to say that the perfect inflection is irrelevant to the meaning. Perfect verbs nearly always carry a strong aspectual component - i.e., the action is completed. Here, the relevance is that the forgiveness is complete at the time that it is realized, whenever that might be. This is known (in some circles) as a proleptic perfect. The timing is dependent on the timing of the protosis. Here the aorist subjunctive ἀφῆτε provides almost no intrinsic time information. Contextually, these are anticipated as future events, consequent to the commissioning and receiving of the holy spirit in the prior verse. 

In summary, there is no "past tense" here. The combination of the verbal inflection and the conditional construction tell us that the forgiveness is complete at the time that the (apostles') forgiving occurs.4 Context tells us that these events are future to the time of Jesus’ speaking. Going beyond that (regarding timing) based on this passage alone seems to me misguided. Others have addressed external considerations that may be relevant.




Notes
 


1. There is a significant text variant here. This perfect quoted here from NA27/28 (=UBS4) was given a {B} ("almost certain") rating by the UBS Committee. The Textus Receptus, with the Byzantine mss, has the present ἀφίενται; Sinaiticus has the future ἀφεθήσεται (reflected also in most of the Latin tradition); the perfect adopted here is supported by Alexandrinus, Bezae, several major families of miniscules, and probably Vaticanus (although spelled differently). This decision was defended by Metzger in his textual commentary on the basis that the present and future are likely scribal simplifications of the perfect (in addition to the external evidence).
  


2. Passionately. This debate is sometimes referred to as the Porter/Fanning debate in recognition of two scholars who published pioneering works in the area. An introduction to the issue by D.A. Carson, published as the first chapter in a book that contained some of the most important evaluations of their theories, is a good place to start if interested. 
  


3. The apodosis is the “then” statement of an “if... then...” contingency. The protasis (“if...”) here is introduced with ἄν, standing for ἐάν, an abbreviation that “is rare in Hellenistic Greek....but appears....John 20:23” (BDAG).
  


4. James Boyer has argued that "all third class conditions are essentially future contingencies". I think it’s safe to say that most people don’t agree with him completely on the all bit, but this is certainly a very common scenario.
  


5. It is, unfortunately, impossible to express "to be complete" and "to occur" in English as a finite verbs without any time implications. Present tense is, by my reckoning, as close as we come, but realize that these may be past, present, or future.