Is acceptance the most important spiritual practice?
Yes. But it’s important not to confuse self-acceptance with acceptance of external circumstances. In spirituality, you do not surrender to external circumstances, you surrender to yourself. Surrendering to external circumstances comes naturally as a consequence.
The more accepting you are of yourself, the more knowing you are of yourself. The ultimate knowledge of yourself is enlightenment, and in the process of self-acceptance you will need to accept it all: your beauty, your ugliness, your love, your hatred, your vanity, your aggression, your fear, your pain, your tears, your anger, your compassion, all of it.
The unconditional love for yourself is the only thing that can possibly allow this to happen. This is the muscle to train. When your unconditional love to yourself becomes absolute, your unconditional love to others will become absolute also, not the other way around.