The Jedi are weak, this much is proven. Though they are many, they lack strength individually. They make up for this individual weakness with numbers.
How do they do this? Conviction. They are truly convinced that their creed and their code are the true way. The only way.
You see, they are weak because they only scratch the surface. They limit themselves in knowledge to only what they can see. They are afraid to delve further, to taste True Power. At times, a few Jedi discover this and seek out Power. They come to the Sith to find it. Some of the greatest Sith were once Jedi, if the scraps of ancient texts I have discovered are true. These Sith tore down the walls the Jedi built to discover what lay beyond.
Sith power. True Power. Absolute Power.
This is only achievable through the Dark Side of the Force.
Some toy with the Darkside. Some play with it, like a child does with a flame. Some are burned by the flame and shy away. Others draw closer, for pain intrigues them. It lures them in. Not to seek it, but to find the cause. To understand what power it is to cause pain. The Dark Side teaches this.
Power is only ever attained through dominance of pain. In dominating pain, we conquer ourselves. In conquering ourselves, we can conquer the galaxy.
Only through conflict can we grow stronger. Only by victory can we apply this strength. A defeated Sith is a broken Sith. Weakness is a sin. Victory is our Creed. It is our motto. It is our life.
Victory is Life. Defeat is Death. There can be nothing else.