The first scripture that occurs to me in this connection is Galatians 1:8, where St. Paul writes: "But though we, or an angel from heaven, preach a gospel to you besides that which we have preached to you, let him be anathema."

(LDS view)

“One Lord, One Faith, One Baptism”

Mormons are strong proponents of the claim that only one denomination of Christianity is recognized by God, and that all other denominations:

"...teach for doctrines the commandments of men, having a form of godliness, but they deny the power thereof." (Joseph Smith History 1:19)

One scripture that they use to support this is Ephesians 4:5

"One Lord, one faith, one baptism,"

There is only one true God, so there can only be one true Church.

An excellent explanation of this scripture is given in Chapter 44 of the New Testament Student Manual:

The word “one” appears seven times in Ephesians 4:4–6. Oneness and unity are important themes in Ephesians and in Paul’s other writings. Paul constantly preached about unity and prayed for unity among Church members (see Romans 12:5; 1 Corinthians 1:10; 2 Corinthians 13:11). In modern times, the Lord revealed to Joseph Smith that unity is a key law of the celestial kingdom (see D&C 105:3–5). There is only one true Lord, one true faith, one true baptism, and one true Father of all.

Elder Delbert L. Stapley (1896–1978) of the Quorum of the Twelve Apostles spoke of the critical role Apostles played in maintaining unity and pure doctrine:

“After Jesus put his apostles in charge of the Church anciently, they preached the same unity of doctrine and practiced the same ordinances which Jesus had given them. …

“… As long as they remained on the earth, functioning under the authority Jesus gave them, unity of doctrine and uniformity of the ordinances prevailed. The gospel message, which they were commanded to take to all the world, was the same to everyone everywhere. People were not taught different gospels and then given a choice. There was only one plan for all.

“Because of the universality of these requirements for salvation, the apostle Paul wrote: ‘There is … one Lord, one faith, one baptism.’ (Eph. 4:4–5.) …

“One church, one authorized ministry, one orthodox gospel doctrine, and one Holy Ghost characterized the church of Jesus Christ in His time. ‘For God is not the author of confusion, but of peace, as in all churches of the saints.’ (1 Cor. 14:33.) Thus God’s revelation to leaders of the Church of Jesus Christ was reasonable, consistent, and unified.

“It was only after the death of Christ’s apostles that revelation ceased. The pure doctrines Christ taught became diluted with the philosophy of the world, and profane innovations appeared in the ordinances of the church. Eventually, that which had once been clear and understandable became mythical and confusing” (“What Constitutes the True Church,” Ensign, May 1977, 22).

21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. 22 Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ 23 Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’” (Matthew 7:21-23, NIV)

A person who claims Jesus as their Lord (ie a Christian) must do the will of Jesus' Father, Almighty God. Even if they do all sorts of great things, if they don't do what the Father says to do, they will be rejected.

Some denominations do certain things based on what they interpret as God's will, so they may believe that other denominations not doing those God-willed things would be rejected, according to this verse.