The tense it is in is Aorist:
The aorist tense is characterized by its emphasis on punctiliar action; that is, the concept of the verb is considered without regard for past, present, or future time. There is no direct or clear English equivalent for this tense, though it is generally rendered as a simple past tense in most translations.
So it must be understood as:
1 John 1:9 If we confess our sins, he is faithful and just to have forgiven, to forgive and always to forgive us our sins, and to have cleansed, to cleanse, and always to cleanse us from all unrighteousness.
and the tense of "confess" is:
Present
The present tense represents a simple statement of fact or reality viewed as occurring in actual time. In most cases this corresponds directly with the English present tense.
Some phrases which might be rendered as past tense in English
will often occur in the present tense in Greek. These are termed historical presents, and such occurrences dramatize the event described as if the reader were there watching the event occur. Some English translations render such historical presents in the English past tense, while others permit the tense to remain in the present.
So, this is referring back to our confession of sins when we came to faith in Christ Jesus.
A similar statement would be in assuring troops of their basic training.
"If we underwent basic training, it will prove itself useful throughout our entire military career."
The example is speaking to troops who obviously went through basic or they would not be troops. The same is true of 1 John 1:9 being written to believers. Another way to say "confess our sins" is to "agree with God about our sins" so that "if" can best be understood as "since."
Note in 1 John 2:1 the phrase "if any man sins..."
The Bible clearly indicates all human beings with the exception of Jesus are sinners.
1 John 1:7 But if {since} we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 8 If we {otherwise} say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If {since} we confess our sins, he is faithful and just to forgive {past present future} us our sins, and to cleanse {past present future} us from all unrighteousness. 10 If we {otherwise} say that we have not sinned, we make him a liar, and his word is not in us.
The gist of the passage is that it is an assurance to believers about the security and totality of forgiveness, and a juxtaposition with not facing up to or owning up to our sin in the first place.
Remember:
Galatians 3:24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. 25 But after that faith is come, we are no longer under a schoolmaster.
Romans 3:31 Do we, then, nullify the law by this faith? Not at all! Rather, we uphold the law.
Don't be ensared by ecclesiastical pharisee-ism.
Ephesians 2:8 For it is by grace you have been saved, through faithand this not from yourselves, it is the gift of God 9 not by works, so that no one can boast. 10 For we are Gods workmanship, created in Christ Jesus to do good works, which God prepared in advance for us to do.
You will find that those who try to hold you to some code of baptism, tithing, confession, etc. are usually boasters about all that they do. It may even start out as a devotion to the Lord but it ends up as a duty to themself. Always. Why? Because man cannot save himself at all. He cannot even participate in his salvation. It's all up to Christ.