For the believers, the Lord is a sure foundation. He is a starting block on the pathway of faith to eternal life. But to the unbelieving and the wicked he is a stumbling block. Instead of gaining great momentum, the wicked trip over the Corner Stone and His word.
"But we preach Christ crucified, unto the Jews a stumbling-block, and unto the Greeks foolishness" (1 Cor. 1:23). How is Christ a stumbling block? In the same way that the story of the coming forth of the Book of Mormon is a stumbling block to people. Or, in the same way that Joseph Smith or the Word of Wisdom or tithing can be stumbling blocks. Sometimes people stumble over not having tangible proof of the gospel; they want proof and a sign. As a missionary I met a man who told me who told me, "I'd believe all this about the Book of Mormon if I could just see the gold plates with my own eyes, or if I could see an angel." He wanted proof that what we were teaching him was true. However, there is an easy way to obtain proof - pray and ask God for the truth (see Moroni 10:3-5). This experience with this man taught me another important truth; the Lord taught that a "wicked and adulterous generation seeketh after a sign" (Matt. 12:39). It turned out that this man had three children with a woman whom he had never married. They were living together but were not married. Was he an adulterer in the strict sense of the word? I don't know, but he was "living in sin" as the saying goes.
What does it mean for Christ to be our Corner Stone? Elder Holland explained one role of the Savior,
"In the words of esteemed New Testament scholar N. T. Wright, 'The risen Jesus, at the end of Matthew's Gospel, does not say, 'All authority in heaven and on earth is given to the books you are all going to write,' but [rather] 'All authority in heaven and on earth is given to me.' In other words, 'Scripture itself points...away from itself and to the fact that final and true authority belongs to God himself.' So the scriptures are not the ultimate source of knowledge for Latter-day Saints. They are manifestations of the ultimate source. The ultimate source of knowledge and authority for a Latter-day Saint is the living God. The communication of those gifts comes from God as living, vibrant, divine revelation." (Elder Holland, Ensign, May 2008).
Christ is the ultimate source of knowledge and truth. The is the source of authority for us as Latter-day Saints. He is the foundation upon which we build.
Link to part 1 of this essay.