the opposite of retain is remit, or forgive.
Remission of sin is received through several means. Simple confession and repentence before God is the most common way. Baptism in water is another way- dealing with the past sins of a new believer. Being released by the laying on of hands of an elder is also a means of remission.
I personally think personal remission can be different from the unilateral forgiveness God has granted all. Sometimes release has been granted, but the prisoner needs a key to open the cell.
So James says,
14 Is anyone among you sick? Then he must call for the elders of the church and they are to pray over him, anointing him with oil in the name of the Lord; 15 and the prayer offered in faith will restore the one who is sick, and the Lord will raise him up, and if he has committed sins, they will be forgiven him. 16 Therefore, confess your sins to one another, and pray for one another so that you may be healed. The effective prayer of a righteous man can accomplish much.
So there is a connection between sickness and sin and healing and forgiveness, or remission of sin. In your example, Mark 2:9, as Jesus said, Judge for yourselves whther it is easier for me to say, "Your sins are forgiven you, or "Rise up and walk
He was showing that healing flows through the forgiveness of sin.
I think John 20:23 is similar to Mark 16:19 19 "I will give you the keys of the kingdom of heaven; and whatever you bind on earth shall have been bound in heaven, and whatever you loose on earth shall have been loosed in heaven."
As in another example Jesus gives, "If he refuses to believe you, take him before the church and let him be as an unbeliever"
or Paul saying, "I have determined, in the presence of God and His holy angels, to deliver such a one over to Satan for the destruction of the flesh...."
IMO, All these kinds of verses relate back to Jesus words in John 20:23.
As we mature in the Spirit of Christ("be careful how you hear, to him who has more shall be given, to him who has not even that which he has shall be taken away") various keys to the kingdom begin to function in our lives as "repairers of the breach"- only because a mature disciple can be trusted to use any gift of the spirit or spiritual key to heal that same separation of connection we are talking about. :o)
So I see it as you do....... we are all connected. Sin separates, forgiveness unites. All sin is ultimately a statement that I am not a part of you, so I am not accountable to you. Sin creates this separation and it is a wound in the whole of mankind.
This separation is what Jesus came to heal, by showing that He was connected to all. Now that healing is spreading to all, one cell at a time, until all are forgiven, until all are healed, until all are one and universally connected.
20 "I do not ask on behalf of these alone, but for those also who believe in Me through their word; 21 that they may all be one; even as You, Father, are in Me and I in You, that they also may be in Us, so that the world may believe that You sent Me."