Stand firm then, with the belt of truth buckled around your waist...
The Belt of Truth
When preparing for battle, the belt would have been the first piece of protective armor put on. It was more than a strap around the waist. It had other leather straps with brass plates connected to them which hung over the groin area of the soldier.
Belts were used to tie up the garments so they would not get in the way while fighting. Without the belt the armor becomes weak and wobbly. The same goes with the truth. If we don't have a definite absolute truth everything else becomes weak, awkward and a burden. The call to already have "buckled" is a call to already be prepared. You start with the truth. You can't go any further if you don't have the truth.
This truth is not relative. This isn't theology or doctrine. It is the one simple truth that everything else hangs on. JESUS. In the movie A Few Good Men Colonel Jessep says, "You can't handle the truth!" I believe when we try to add so many other things (theologies and doctrines) to the truth of Jesus whether they're indeed the truth or not it begins to make the truth harder to handle. The person of Jesus begins to get blurred. We are meant to be replicas of Jesus and a replica is no good if it has things added to it based on a belief about the original.
Jesus said that He is the truth. There is no other truth besides Him.
In our faith we cannot get anything else right if our understanding of Jesus is askew. It doesn't matter what you believe about the rapture, if you are pre-trib, mid-trib, post-trib, or about speaking in tongues, and all the other debatable topics. They come second to Jesus and a far second at best.
We cannot do Christianity right with a false Christ. An image of a true Christian is found in Christ. We find our purpose, our mission, our hope, our joy, our peace, our freedom and our life in Jesus.
So make sure the belt is tight and you'll be able to stay in the fight.