How much will the owner’s belief quotient and partnership status detract from the power of the blessing conferred? If a priest blesses the car of an atheist husband at his wife’s request, and the husband is never told, what are the likely long-term benefits, if any?

If the husband is a nonbeliever but the wife is, her intention will be perfectly satisfactory to confer the blessing for his vehicle and all that takes place with him present. That is not to say he cannot be a negative influence. Human intentions are always the most important considerations and will outrank divine will because of the ongoing paradigm of free agency and free will being absolute as benefits and conditions of human existence. So even driving the blessed automobile will not protect a true believer or an atheist if they take reckless action or neglect safety precautions by engaging in speeding on an icy road or imbibing alcohol before the journey and being impaired mentally. To some extent, such events can be lessened in potential consequences, but this may not be total. Oftentimes there are karmic liabilities present from many past experiences and the Law of Karma may well identify an opportunity to cause some problem and this may override the benefit of the blessing. So this makes a good point, that it is best to not rely on a single holy ritual to think of it as an insurance policy and a guarantee of safety and well‑being and ignore sources of risk and see to good self-care, common sense, and following good sound practices in one’s conduct.