First we must know that we should be just one man united with the one God but, because of Adam’s fall, we are many separated between us and separated from the Holy Trinity. This generated a chaotic behavior inside and outside of us – and among us, something which (if isn’t cured) leads to the eternal hell.
That’s why God gave us this great gift called “time” which is our possibility to regain our unity with God and as a result between our fellow man.
The practical way to regain this unity is via obedience, that is the accord of our free will in God.
The blessing of an institution closer to God than us (being it a priest, bishop, elder, abbot etc.) is the concrete expression of his will (and God’s will) via words that his will is in accord with our will – with what we do. In many languages, the word „blessing” means „good word(ing)” – ευλογία (Greek), binecuvântare (Romanian), benedixitque (Latin), благословение (Russian). So, when we have the blessing we have the „good wording”, that is the approval of the will of the institution of God since He said „For where two or three are gathered in my name, there am I with them” (Matthew 18:20)
The sin is practically disobedience, separation, and ultimate death.