Character is an essential principle for sustained spiritual practice. 

The analogies given to us are that unless we become more humble like the grass, tolerant and forgiving like the tree, and unless we can joyfully offer respect to others without demanding it for ourselves, it is very difficult  to chant the holy names of God continuously and with great feeling. 

Even if we follow all the regulative principles and cut the weeds in our heart or perform our spiritual practices and cultivate deep knowledge, we won’t make genuine spiritual progress if there is still a basis of false pride underneath it all. 

We can very intellectually, philosophically, scientifically and technologically understand what is the difference between matter and spirit, what is truth and what is illusion, but if the root of our ego remains then it’s just a matter of time until the weeds keep coming back. This is why the principle of humility is at the very core of the science of self-realization.

Similar to if you have a disease in the blood, that disease may manifest as rashes and boils and so many other discomforts externally. There are medical processes to cure the individual symptoms and gain a temporary relief from the pain and agitation, but as long as the underlying disease is still there in the blood, then the symptoms will continue to resurface over and over again. And sometimes the more you deal with the symptoms without dealing with the cause, when the symptoms come back they get worse and worse each time.  

We have examples in our Vedic histories like Hiranyakashyapu and Ravana, both of whom knew so much Vedic philosophy and performed severe austerities and sacrifices. They could speak about the difference between body and spirit better than all the learned pandits today, but they had false pride and didn’t carry the mood of humble service. Therefore all their austerities and all their cultivation of knowledge and all their sacrifices and everything else they did caused them to seem very powerful and liberated, but they weren’t. They were entrapped by the weeds which kept them from experiencing genuine love and devotion to God and others.