Rabbi Nechemiah teaches a principle of extraordinary generosity. If a person takes upon himself even a single mitzvah in true faith, that person is worthy of having the Holy Spirit — the Ruach (spirit) HaKodesh — rest upon him. Not all 613 commandments. Not a lifetime of perfect observance. One mitzvah, performed with genuine faith, is enough to open the gates of divine inspiration.
The proof comes from three of the greatest figures in Jewish history. Moses, David, and Deborah all chanted song before God, and the Holy Spirit rested upon each of them. What did they have in common? Each one, at a crucial moment, acted from a place of complete faith. Moses led the people in the Song of the Sea after the crossing of the Red Sea. David composed psalms that became the permanent liturgy of Israel. Deborah sang her victory song after the defeat of Sisera's army.
The Mekhilta's teaching democratizes prophecy. Rabbi Nechemiah is not saying that only exceptional individuals like Moses and David can receive the Holy Spirit. He is saying the opposite — that the same spiritual power that rested upon the greatest prophet and the greatest king is available to any person who performs even one commandment with wholehearted faith. The barrier to divine inspiration is not talent or lineage or scholarship. It is sincerity. The Holy Spirit does not count how many mitzvot (commandments) a person has fulfilled. It responds to the quality of faith behind even a single act of devotion.