Hello again ForAllTruth, the reason that I asked you whether or not you believed the Apostle John was regenerate at the time of the writing of 1 John is because of the inclusion of himself in the following passage (among many others). For what it's worth, I believe that he was the very first NT (regenerate) believer following the Lord's resurrection .. see
John 20:8.
As far as it being possible for a true/regenerate believer to be
simul justus et peccator (as Martin Luther famously said), I'll stick to 1 John to consider that possibility for right now.
Please take special note of the words in bold below, as they tell us who the Apostle was both writing to and/or about concerning sin (as well the important use by the Apostle of the Greek present tense and what that helps us understand about what is being said here).
1 John 1
8 If we say that we have no sin, we are deceiving ourselves and the truth is not in us.
9 If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness.
10 If we say that we have not sinned, we make Him a liar and His word is not in us.
1 John 2
1 My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an Advocate with the Father, Jesus Christ the righteous;
2 and He Himself is the propitiation for our sins; and not for ours only, but also for those of the whole world.
3 By this we know that we have come to know Him, if we keep His commandments.
4 The one who says, “I have come to know Him,” and does not keep His commandments, is a liar, and the truth is not in him;
5 but whoever keeps His word, in him the love of God has truly been perfected. By this we know that we are in Him:
6 the one who says he abides in Him ought himself to walk in the same manner as He walked.
7 Beloved, I am not writing a new commandment to you, but an old commandment which you have had from the beginning; the old commandment is the word which you have heard.
8 On the other hand, I am writing a new commandment to you, which is true in Him and in you, because the darkness is passing away and the true Light is already shining.
9 The one who says he is in the Light and yet hates his brother is in the darkness until now.
10 The one who loves his brother abides in the Light and there is no cause for stumbling in him.
11 But the one who hates his brother is in the darkness and walks in the darkness, and does not know where he is going because the darkness has blinded his eyes.
12 I am writing to you, little children, because your sins have been forgiven you for His name’s sake.
13 I am writing to you, fathers, because you know Him who has been from the beginning. I am writing to you, young men, because you have overcome the evil one. I have written to you, children, because you know the Father.
14 I have written to you, fathers, because you know Him who has been from the beginning. I have written to you, young men, because you are strong, and the word of God abides in you, and you have overcome the evil one.
In quick summary, the above was written
to believers at
all levels of spiritual maturity in the church. In fact, the
(now elder) Apostle John includes ~
himself~ in the mix of those who need to continually confess their ongoing sins to God whenever they commit them (to be
forgiven and
cleansed of them so that full fellowship w/God and the
(as King David put it) "
joy of Thy salvation" ..
Psalms 51:12, can be restored to them).
We are also told that "
we have (present and ongoing)
an Advocate" ..
1 John 2:1 who speaks to the Father on our behalf whenever we sin (and that He is/will always be, present and ongoing,
the propitiation for the sins that we commit ..
1 John 2:2).
I'll stop here (as this is more than plenty to consider for now

).
The next thing is to move on to why 1 John 3 does not contradict what is said above in 1 John 1/1 John 2, as well the additional, Biblical evidence (the Bible is replete with it) concerning the ongoing sins of true believers (and what we are commanded to do about them, to avoid them or to remedy them).
God bless you!!
--David
p.s. - how a person (who claims to be a believer) responds to his/her ongoing sin is a big part of the evidence that tells us if they are ~truly~ "
in Christ" (and thereby, that their "claim" is authentic), or if they are simply/sadly CINO instead. This is, in point of fact, part of the test that we are admonished to take (
2 Corinthians 13:5) that helps us know the truth about our own salvation
As theologian/pastor A. W. Pink once said,