Bhagavad Gita, Chapter 4 verse 1: Lord Krishna: I taught this imperishable Yoga to Vivaswat, Vivaswat declared it to Manu; Manu told it to Ikshwaku. 

The“ kingdom of heaven lies within you.” — Luke, 17:21

Here, ‘Yoga’ means karma yoga, the yoga of action to renounce action.  When practicing karma yoga we act to serve the desires of others and break the cycle of impression-desire-action at the level of our desire to find happiness and contentment in the material world outside ourselves*. No personal desire, no action to achieve the desire. Our outward flow of attention to seek happiness “out there” stops dead in its tracks; drawn by the kingdom of heaven the mind turns within and we find happiness and contentment where it has always been, deep within (2.45). 

This practice of karma yoga is ‘imperishable’ because we are imperishable. From verse 2.12, ‘There never was a time when I was not, nor you, nor these rulers of men. Never will there be a time when all of us cease to be.’ We are imperishable, eternal and outside the passage of time. 

 In our imperishable nature, we were created along with the  practice of karma yoga, to serve the desires of others. From 3.10,  ‘In the beginning, having created man along with Sacrifice, the Lord of Creation said: By this Sacrifice [of desire] ye shall prosper and this shall bring forth the fulfillment of desire.’ That is, we were created along with renunciation of desire, which lies at the heart of practicing karma yoga. We are inseparable from the practice. The practice of innocently renouncing desire by serving others’ desires is in our DNA; from the ‘beginning’ our very nature is to serve. Serving the desires of others is who and what we are. We are ‘imperishable’ and therefore the practice of karma yoga, which is inherent to us, defined by our nature and our duty to perform (3.08}, is also ‘imperishable.’

*impression-desire-action: Through attractive qualities found in objects of the senses we get stuck in the cycle of impression-desire-action. Say we see gain in the happiness a spiffy new Porsche promises; we desire the Porsche; we take action to acquire it; in time the newness of the Porsche wears off, leaving us with an impression of dissatisfaction. We see happiness anew in a different object of the senses  — say, a new pair of running shoes, which in time wear thin— and repeat the cycle of impression-desire-action … ad nauseum. We become bound to our actions to achieve lasting happiness and contentment in objects of the senses. (For a detailed discussion on bondage to action see 2.39 and 2.45 on how to break with it.)