Follow the use of “we” throughout the epistle.
It is important to consider who is included when the word "we" is used. If we include all believers, we need to ask ourselves why "we" don't move mountains. have every prayer answered by God, and sell everything we have and hold all things in common.
If by "the epistle" you mean 1 John, where does that epistle mention moving mountains, having every prayer answered by God, or selling everything we have? Faith moving mountains is in Matthew's gospel:
(Mat 17:20 NKJV) So Jesus said to them, "Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible for you.
Jesus says the same thing in Matthew 21:21. Where do we read of
any believers moving actual mountains?
Having all things in common is something we find in Acts:
(Act 4:32 NKJV) Now the multitude of those who believed were of one heart and one soul; neither did anyone say that any of the things he possessed was his own, but they had all things in common.
That there was no compulsion about this is seen when we read further in the chapter, and find Peter's words to Ananias and his wife Sapphira, who sold a piece of land, and claimed to be bring the whole of the proceeds to the apostles:
(Act 5:3-4 NKJV) But Peter said, "Ananias, why has Satan filled your heart to lie to the Holy Spirit and keep back part of the price of the land for yourself?
While it remained, was it not your own? And after it was sold, was it not in your own control? Why have you conceived this thing in your heart? You have not lied to men but to God."