I would say the most important word in scripture that is overlooked way too often is the two lettered word " IF "
The word "if" doesn't hold any power over other words in Scripture.
Way to often when someone enters a church, their handed a Bible and told to accept Jesus and everything in that Bible is yours. Thats not true.
Only when someone, a called out one responds to the call of the Father with pisteuo, a personal surrender to Him and a life inspired by such surrender, does that Bible become theirs.
No other word in the Bible holds that kind of power over all the other words in scripture.
Imagine the Bible and everything in it as being a house with a locked door in front. To get in that house, the key to open the door is only one thing, one word, pisteuo used 248 times in the NT.
Once that key is inserted and turned by a personal surrender to Him and a life inspired by such surrender, it allows God the Father to either accept or deny the surrendered life. If accepted, the door opens and we take steps into that house, each step being a better surrender to Him. At this point we are positioned correctly to receive the Spirit of Christ and become His.
No ther word can facilitate this process, only pisteuo or a continually surrendered life.
If anyone is attempting to open the door in any other way than a personal surrender to Him and a life inspired by such surrender, you'll simply be standing at the door while God waits for the correct key to be used.