In a large bowl, whisk together the tequila, lime juice, olive oil, honey, minced garlic, cumin, chili powder, salt, black pepper, and red pepper flakes (if using) until well combined.
Add the chicken breasts to the marinade, ensuring they are fully coated. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, or up to 4 hours for more flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade and let any excess marinade drip off. Discard the leftover marinade.
Grill the chicken for 6-7 minutes on each side, or until the internal temperature reaches 165°F.
Once cooked, remove the chicken from the grill and let it rest for 5 minutes before slicing.
Garnish with fresh chopped cilantro before serving.