Ephesians 4:30
King James Version
30 And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.
I pay much more attention to the "grieve not the Spirit" part of this, rather than what "sealed" means theologically. Grieving the Spirit is something much more visceral, something felt as an internal conflict and discomfort. I take that to mean to go against that higher consciousness made awake in us by God.
When we are convinced that something is wrong, and we choose to act against that conviction because we are driven by lower impulses and urges instead, i.e, "I just have to call them a loser, because I'm mad and I'm justified in saying so!", there is that calm voice inside us which knows better, but we choose to act against that. To me, that is "grieving" that Spirit.
And the end effect of doing that all the time, that inner higher voice becomes weakened because we are paying more attention to the 'flesh' and its urges, instead of the Spirit and its Wisdom and restraint. There's quote from Sitting Bull that goes like this:
"Inside of me there are two dogs. One is mean and evil and the other is good and they fight each other all the time. When asked which one wins I answer, the one I feed the most.”
I believe that's what this verse is admonishing Christians to do. Don't grieve the Spirit, by suppressing it within us, thereby harming our spiritual life. That is why Paul continues,
"Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice. 32 Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you."
"Whichever one I feed the most wins".