I was answered that I must join none of them, for they were all wrong; and the Personage who addressed me said that all their creeds were an abomination in his sight; that those professors were all corrupt; that: “they draw near to me with their lips, but their hearts are far from me, they teach for doctrines the commandments of men, having a form of godliness, but they deny the power thereof.” (Pearl of Great Price, Joseph Smith—History, Joseph Smith—History 1)
When 14-year-old Joseph Smith went into what is referred to now as the Sacred Grove to pray to pray concerning the welfare of his soul and to ask which church to join. Joseph Smith received a marvelous and miraculous heavenly visitation of God the Father and Jesus Christ.
During this heavenly visitation Joseph Smith asked which church he should join. His Christian father was disallusioned by organized religion. His mother and siblings were drawn to the Presbyterian while he preferred the Methodist. Christ's response surprised Joseph. Never before had he considered that all the churches were in error. One reason Christ gave for their error was that they "denied the power of godliness".
What is the power of God? What does it mean to deny the power of godliness? How do other churches deny the power of God?
Power is a physics principle, specifically the ability to do work. Power equals work divided by time. Work is also a principle of physics and natural law, specifically force times time. However, the interesting aspect of work is that if you push against a wall, you may have exerted a lot of effort and energy; but, if the wall doesn't move, then no work has been performed. And if no work has been performed, there is no power.
False religion likes to talk about the Atonement of Christ that is all grace without power. Power to do what? Power to repent, power to change, power to receive personal revelation from God, to confer priesthood authority, to experience miracles, to continue to call living prophets, apostles, and seventies, to build sacred temples, and for husbands and wives to be married in this life and to continue to be sealed together for eternity.
For many religions, it is enough to want to repent and change and resist the natural man without being given hope that they can actually do it. For many religions it is enough to ponder the meaning and purpose of life without actually expecting to receive an answer. However, in the true Church of Christ, Christ's Atonement has the power to transform us, sanctify us, perfect us in Christ and reveal to us the answers to the important questions of the soul. The atonement of Christ is power to divide the seas and move mountains, even personal ones.