Jesus the Cornerstone versus the Claims of Mormonism

Mormons refer to their organization as restored Christianity, because, they claim, the original Christianity of Christ and the Apostles was lost. Then they refer to the creeds of all other professing Christians as "abominations." 

There are many biblical reasons for saying that a general apostasy is impossible. The main Scriptural evidence of that is Matthew 16:18, where Jesus says, "On this rock I will build My church, and the gates of Hades shall not prevail against it." The rock is Himself, the chief cornerstone and foundation of the church (Ephesians 2:20). If the true church disappeared, then it can only be said that the cornerstone had failed. 

But I want to turn to a different verse: "'And as for Me, this is My covenant with them,' says the LORD: 'My Spirit that is upon you, and My words that I have put in your mouth, shall not depart out of your mouth, or out of the mouth of your offspring, or out of the mouth of your children’s offspring,' says the LORD, 'from this time forth and forevermore'" (Isaiah 59:21).

Here, Jehovah, the preincarnate Christ, includes among His covenant promises that His words will never completely disappear from among His people. Is that not exactly what an apostasy would be? 

The message of that verse is exactly the same as that in Matthew: Jesus makes it His personal guarantee that His Gospel, by which His church is gathered, will never depart completely from among men. That is exactly the opposite of the claims of the Mormon organization. "Let God be true though every one were a liar, as it is written, 'That you may be justified in your words, and prevail when you are judged'" (Romans 3:4).