Centuries ago, Daniel and his young associates were suddenly thrust from security into the world—a world foreign and intimidating. When Shadrach, Meshach, and Abed-nego refused to bow down and worship a golden image set up by the king, a furious Nebuchadnezzar told them that if they would not worship as commanded, they would immediately be cast into a burning fiery furnace. “And who is that God that shall deliver you out of my hands?”
The three young men quickly and confidently responded, “If it be so [if you cast us into the furnace], our God whom we serve is able to deliver us from the burning fiery furnace, and he will deliver us out of thine hand.” That sounds like my eighth-grade kind of faith. But then they demonstrated that they fully understood what faith is. They continued, “But if not, … we will not serve thy gods, nor worship the golden image which thou hast set up.” Dan. 3:18 That is a statement of true faith.
They knew that they could trust God—even if things didn’t turn out the way they hoped. They knew that faith is more than mental assent, more than an acknowledgment that God lives. Faith is total trust in Him.
Faith is believing that although we do not understand all things, He does. Faith is knowing that although our power is limited, His is not. Faith in Jesus Christ consists of complete reliance on Him.
Shadrach, Meshach, and Abed-nego knew they could always rely on Him because they knew His plan, and they knew that He does not change. They knew, as we know, that mortality is not an accident of nature. It is a brief segment of the great plan of our loving Father in Heaven to make it possible for us, His sons and daughters, to achieve the same blessings He enjoys, if we are willing.
They knew, as we know, that in our premortal life, we were instructed by Him as to the purpose of mortality: “We will make an earth whereon these may dwell; And we will prove them herewith, to see if they will do all things whatsoever the Lord their God shall command them.”
So there we have it—it’s a test. The world is a testing place for mortal men and women. When we understand that it’s all a test, administered by our Heavenly Father, who wants us to trust in Him and to allow Him to help us, we can then see everything more clearly.
His work and His glory, He told us, is “to bring to pass the immortality and eternal life of man.” He has already achieved godhood. Now His only objective is to help us—to enable us to return to Him and be like Him and live His kind of life eternally.
Knowing all this, it was not difficult for those three young Hebrews to make their decision. They would follow God; they would exercise faith in Him. He would deliver them, but if not—and we know the rest of the story. (Dennis E. Simmons, “But If Not …,” Ensign, May 2004, 73. Bold added by Rich)
There is another GC article that also references "But if not".
Too often we offer our prayer or perform our administration and then wait nervously to see whether our request will be granted, as though approval would provide needed evidence of His existence. That is not faith! Faith is, quite simply, a confidence in the Lord. In Mormon’s words, it is “a firm mind in every form of godliness” (Moro. 7:30; emphasis added). The three Hebrew magistrates expressed trust that the Lord would deliver them from the fiery furnace, “but if not,” they said to the king, “we [still] will not serve thy gods” (Dan. 3:18; emphasis added). Significantly, not three but four men were seen in the midst of the flames, and “the form of the fourth [was] like the Son of God” (Dan. 3:25). (Lance B. Wickman, “But If Not,” Ensign, Nov 2002, 30)Update 2011-05-17
As I was searching for this talk I found out that Martin Luther King Jr. had given a speech on was seems like the same theme.