Needlessly transgressing the Pratimoksha or Bodhisattva precepts.
If we think that since we are now Tantric practitioners we can ignore our Pratimoksha or Bodhisattva vows, we incur a gross downfall.
In the Dharma we say there are two types of cause: substantial and circumstantial. The substantial cause is the thing that transforms into the next thing, for example the acorn transforms into the oak tree. The circumstantial causes are the causes and conditions which facilitate that transformation, such as sunlight, water and soil. If there is no acorn, no amount of sunlight, water and soil will produce an oak tree. Likewise, without sunlight, water and soil the acorn will never transform into a tree. Both are necessary.
We can also trace the continuum of substantial and circumstantial causes beyond a single transformation. For example, we can say the oak tree is the substantial cause of my oak furniture, and the circumstantial causes are the carpenters and their tools which shaped them. If later there was a fire at my house and the oak furniture burned, the substantial cause of the flames would be the wood in the furniture and the circumstantial causes would be whatever ignited the fire and the oxygen in the room. In this way, we see the transformation of the acorn into the oak tree into the furniture into the flames. If there was no acorn, oak tree, or furniture, there could be no flames.
In exactly the same way, the substantial cause of a mind keeping the tantric vows purely is a mind keeping the bodhisattva vows purely. The substantial cause of a mind keeping the bodhisattva vows purely is a mind keeping the pratimoksha vows purely. The substantial cause of a mind keeping the pratimoksha vows purely is a mind keeping the refuge vows purely. Without the acorn of our refuge vows we can’t have the oak tree of our pratimoksha vows, the furniture of our bodhisattva vows or the wisdom flames of our tantric vows. If we understand this clearly, there is little danger of our finding Sutra and Tantra to be contradictory. It is on the foundation of seeking refuge, striving for liberation, questing to become a Buddha that we forge ourselves with Tantra into our highest yoga tantra yidam.
What this means in practice is if we are transgressing our lower vows, we are necessarily not keeping purely our higher vows. The qualification of our practice of the higher vows will never outstrip the qualification of our practice of the lower vows. If we want to experience the fruit of our tantric vows, we must first cultivate the crops of our refuge, pratimoksha and bodhisattva vows.
The circumstantial causes of all of these transformations is our practice of the vows themselves. When you take the substantial cause of a root mind and you add the circumstantial causes of the practice of the refuge vows, our root mind transforms into a mind that is keeping the refuge vows purely. If you take that mind and then add practice of the pratimoksha vows, our mind keeping the refuge vows purely transforms into a mind keeping the pratimoksha vows purely. If to that mind you add the practice of the bodhisattva vows, you get a mind keeping the bodhisattva vows purely. If to that mind you add the practice of the tantric vows, you get a mind keeping the tantric vows purely. If you add to that the circumstantial causes of the practices of generation stage and completion stage, you get a fully enlightened highest yoga tantra deity.
Seen in this way, we see there is no enlightenment without the practice of all of our vows and commitments. Practicing our vows and commitments is the very means by which we build enlightenment within our mind.