First of all this is what the word says about what was received in the upper room:
Act 2:38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
39 For the promise is unto you, and to your children, and to all that are afar off, [even] as many as the Lord our God shall call.
This promise holds true for all believers today, not just for a select few.
There were many more than just the twelve in the upper room on the day of pentecost. Is God still calling men. The promise is for as many as the Lord our God shall call. Can we deny that men are still being called?
2Pe 1:10 Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:
Now about remmitting and retaining sins. I have had this much in my thoughts lately as well. Here is what the Lord showed me:
Luk 23:34 Then said Jesus, Father, forgive them; for they know not what they do. And they parted his raiment, and cast lots.
Act 7:59 And they stoned Stephen, calling upon [God], and saying, Lord Jesus, receive my spirit.
60 And he kneeled down, and cried with a loud voice, Lord, lay not this sin to their charge. And when he had said this, he fell asleep.
2Ti 4:16 At my first answer no man stood with me, but all [men] forsook me: [I pray God] that it may not be laid to their charge.
Jhn 20:23 Whose soever sins ye remit, they are remitted unto them; [and] whose soever [sins] ye retain, they are retained.
Vines Dictionary says of this word remit:
signifies "to send away, set free;" also "to let alone,"
Mat 6:15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.
We certainly have the power to forgive sins, if we don't we have the consequences stated here in Matthew 6:15. Not the sort of thing we need to be disobedient to.
1Jo 4:17 ¶ Herein is our love made perfect, that we may have boldness in the day of judgment: because as he is, so are we in this world.
18 There is no fear in love; but perfect love casteth out fear: because fear hath torment. He that feareth is not made perfect in love.
19 We love him, because he first loved us.
20 ¶ If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?
21 And this commandment have we from him, That he who loveth God love his brother also.
Luk 11:2 And he said unto them, When ye pray, say, Our Father which art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done, as in heaven, so in earth.
Luk 11:4 And forgive us our sins; for we also forgive every one that is indebted to us. And lead us not into temptation; but deliver us from evil.
Mat 6:12 And forgive us our debts, as we forgive our debtors.
This is what that word debtors means in the Strongs Concordence in the greek:
1) one who owes another, a debtor
a) one held by some obligation, bound by some duty
b) one who has not yet made amends to whom he has injured:
1) one who owes God penalty or whom God can demand punishment as something due, i.e. a sinner
Mat 6:15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.
Jesus, our example, forgave sins and riled up the religious folks of His day while he walked the earth. He has told us to go and do likewise. When we forgive those who sin against us, we are following His command and those sins are forgiven.