Joseph Smith taught that "faith is the assurance which men have of the existence of things which they have not seen, and the principal of action in all intelligent beings. . . . But faith is not only the principle of action, but of power also, in all intelligent beings, whether in heaven or on earth." (Lectures on Faith, 1:9, 13) In other words, faith encompasses two very disparate concepts; one dealing with our physical reactions to belief in things not seen, and another dealing with our ability to physically harness the power of this belief. I do not claim to know every facet with regard to our exercise of faith, but here are a few of my thoughts:
1) Alma teaches us that "if a man knoweth a thing he hath no cause to believe, for he knoweth it." (Alma 32:18) He further teaches that when a man's "knowledge is perfect" in some thing, his "faith is dormant." (Alma 32:34) In this life, we live by faith; faith in the Lord Jesus Christ and His atoning sacrifice; faith in God's plan and the assurance of exaltation. Whether in this life or the afterlife, when a man obtains a perfect knowledge of these things, does he continue to exercise faith in them as a principle of action?
2) The brother of Jared possessed great faith that the Lord was able to simply touch 16 small stones to make them provide light for his 8 vessels. "Behold, O Lord, thou canst do this. We know that thou art able to show forth great power, which looks small unto the understanding of men." (Ether 3:5) Accordingly, the brother of Jared acted on his faith. But notice what happens next.
The veil was taken from his eyes and the brother of Jared sees the finger of the Lord, and subsequently sees the Lord Himself. The Lord explained that "never has man come before me with such exceeding faith as thou hast." (Ether 3:9) In other words, the brother of Jared's faith was so refined that it allowed him to pass through the veil and obtain a perfect knowledge of our Savior. "Because thou knowest these things ye are redeemed from the fall; therefore ye are brought back into my presence." (Ether 3:13)
The Lord then proceeds to explain this concept. "And never have I showed myself unto man whom I have created, for never has man believed in me as thou hast." (Ether 3:15) Of course Christ had shown himself to others before. But he had never before revealed himself like this. Because of the brother of Jared's great faith, he could not be kept within the veil (see vs. 19-20). Thus, Christ was compelled to reveal himself unto man at this time, where as before, it was Christ's will whether to reveal himself or not.
Note the language in Ether 3:19, "he had faith no longer, for he knew, nothing doubting." In my opinion, this is truly the culmination of our faith; that we may one day see with our natural eye those things which we have seen for so many years with our eye of faith.
3) I believe that receiving the Second Comforter, as spoken of in John 14, and having our calling and election made sure in this lifetime are a direct result of continued faith as a principle of action. This reception may furthermore also be tied to the oft misunderstood ordinance of the second anointing. As LDS, we understand that the Second Comforter is "no more nor less than the Lord Jesus Christ Himself." (TPJS, 149-151) A person who receives the Second Comforter "will have the personage of Jesus Christ to attend him, or appear unto him from time to time, and even He will manifest the Father unto him, and they will take up their abode with him . . . the Lord will teach him face to face, and he may have a perfect knowledge of the mysteries of the Kingdom of God." (Id.)
Does this person, now having a perfect knowledge of Jesus Christ and the "mysteries of the Kingdom of God," subsequently require faith in these things? What of the Celestial Kingdom inhabitants who live with God and Jesus Christ, do they require faith in them anymore? Is this what John meant when he defined eternal life (God's life) as gaining a perfect knowledge of God and His Son? That we increase in knowledge og God through faith as a principle of action?
Faith as a principle of power, I believe, must be eternal since it is "the principle by which Jehovah works, and through which he exercises power over all temporal as well as eternal things." (Lectures on Faith, 1:16) I believe that we, as joint-heirs with Jesus Christ, will use this principle of power throughout eternity.
But, does our faith as a principle of action cease to exist when we gain an intimate and personal knowledge of eternal truths? For that matter, does God exercise faith as a principle of action? And if so, in what does He have faith?