Resistance correctly applied is a key to spiritul growth. There is a lot of students I have taught Kabbalah who have had problems understanding this idea of resistance. For one thing, we tend to put a negative connotation on this word. The type of resistance of which Kabbalah speaks, though, is purely positive, in fact wonderful!
Resistance is all about going against any force which holds us down or back. Resistance is what balances the yin and the yang. for instance, water is yin and fire is yang. Put them together and either the water evaporates, if the fire is stronger at the time, or the fire is put out, if the water is stronger at the time. But lets put some resistance into the equation, lets put a steel cooking pan between them, something to resist the fire totally evaporating the water, and to resist the water spilling onto the fire and putting it out. Now we're cooking!
Resistance is not saying something negative when its come to mind and you have the impulse to do so. Resistance is not eating that piece of chocolate cake just because it's there. Standing up is an act of resisting gravity. Maybe you know you need to meditate, but it's easier to just turn on the T.V. Can you resist the impulse to do so, and go ahead and meditate? Resistance engaged in at the right place and time is what makes your spiritual light burn brighter.
One of the most important forms of resistance is that which occurs inwardly -- resisting the impulse to place your consciousness in the past or future. The only way to do this is to be in the NOW, to place your awareness in the moment. This act of resistance, repeated over and over can eventually clear all karmas and lead us into higher and higher states of consciousness.