"A stumbling block is 'an impediment to belief or understanding' or 'an obstacle to progress.' (Merriam-Webster’s Collegiate Dictionary) To stumble spiritually is 'to fall into sin or waywardness.' A stumbling block can be anything that distracts us from achieving righteous goals.
"We cannot afford to have our testimonies of the Father and the Son become confused and complicated by stumbling blocks. We cannot fall into that trap. Our testimonies of Them need to remain pure and simple....
"If we are to be valiant in our testimony of Jesus, we must avoid the stumbling blocks that entrap and impede the progress of many otherwise honorable men and women. Let us determine to always be in His service. While seeking knowledge, we need to avoid the philosophies of men that lessen our commitment to the Savior. We must see sin in its true light and accept the Savior’s Atonement through repentance. We need to avoid looking beyond the mark and focus on Jesus Christ, our Savior and Redeemer, and follow His doctrine....
"One writer suggests that stumbling blocks may be made into 'stepping-stones to a noble character and to Heaven.' (Henry Ward Beecher)
"For us, being valiant in our testimony of Jesus is a stepping-stone toward qualifying for the Savior’s grace and the celestial kingdom. Jesus Christ is the only name under heaven by which we may be saved. (See 2 Nephi 31:21; Mosiah 3:17.)"
- Quentin L. Cook, "Valiant in the Testimony of Jesus," General Conference, October 2016
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The path through mortality is hard enough without adding the complications of "stumbling blocks," which Elder Cook identifies as the things that distract us from achieving righteous goals. It's particularly concerning when those things impact our testimonies; a personal testimony should "remain pure and simple" and not be complicated or compromised by sophistry, distractions, and mistakes.
Elder Cook describes in this article some of the major obstacles that threaten to disrupt our progress, and encourages us to take appropriate measures to avoid them or counteract them. These include the "philosophies of men," refusal to acknowledge sin, and "looking beyond the mark."