The importance of following the four regulative principles is explained in the following purport by Srila Prabhupada.
We should always remember that to become a bona fide devotee of the Lord is not an easy task, but in this age, by the mercy of Lord Caitanya, it has been made very easy. But if we do not follow even the liberal instructions of Lord Caitanya, how can we expect to discharge our regular duties in devotional service? It is not possible in this age to follow Dhruva Maharaja in his austerity, but the principles must be followed; we should not disregard the regulative principles given by our spiritual master, for they make it easier for the conditioned soul.
As far as our ISKCON movement is concerned, we simply ask that one observe the four prohibitive rules, chant sixteen rounds and, instead of indulging in luxurious eating for the tongue, simply accept prasada offered to the Lord. This does not mean that with our fasting the Lord should also fast. The Lord should be given foodstuff which is as nice as possible. But we should not make it a point to satisfy our own tongues. As far as possible we should accept simple foodstuff, just to keep the body and soul together to execute devotional service.
It is our duty to remember always that in comparison to Dhruva Maharaja we are insignificant. We cannot do anything like what Dhruva Maharaja did for self-realization because we are absolutely incompetent to execute such service. But by Lord Caitanya's mercy we have been given all concessions possible for this age, so at least we should always remember that neglect of our prescribed duties in devotional service will not make us successful in the mission we have undertaken. It is our duty to follow in the footsteps of Dhruva Maharaja, for he was very determined. We should also be determined to finish our duties in executing devotional service in this life; we should not wait for another life to finish our job.
SB 4.8.72
