Imamate is one of the fundamental Shiite beliefs and involves several issues including the necessity of appointing or installing an Imam. Imamiyyah theologians, through various proofs, believe in the rational obligation of God’s appointment of an Imam, one of which is the “proof of the necessity to preserve and clarify the Sharia.” This proof has faced objections, to which theologians and thinkers of Imamiyyah have responded. Using a rational-analytical method, this article presents the proof, answers five objections raised in the book “Theory of Imamate in the Scale of Criticism,” and demonstrates that considering the finality of Islam, after the Prophet’s passing, the Sharia necessarily requires a preserver and clarifier. No one except the infallible Imam can undertake this responsibility. Therefore, for every era, the appointment of an Imam by God is obligatory. This proof and other rational arguments do not express opinions regarding the number of Imams, nor is the occultation of the Imam incompatible with this proof; because the Imam can fulfill the responsibility of preserving the Sharia through means consistent with the philosophy of occultation.