Related Posts: Why Covenants?; Grace; Election; Faith and Charity; Faith and Justification
The word justify can mean (1) innocence before the law, (2) reconciliation with God, and (3) to be shown to be correct (vindication). Unless otherwise stated the word justification is used in the sense of (1), innocence before the law.
Justification
The term justification generally can be thought of as the language of the courts. For example, if the outcome of a trial is decided in your favor you have been justified. This is the context which Isaiah uses:
All the nations have gathered together so that the peoples may be assembled. Who among them can declare this and proclaim to us the former things? Let them present their witnesses that they may be justified, Or let them hear and say, “It is true.” (NASB, Isa. 43:9)
The opposite of justification is condemnation: “by thy words thou shalt be justified, and by thy words thou shalt be condemned” (Matthew 12:36-37; See also Alma 41:15).
In the Mormon view works alone cannot justify us. The reason is partly due to our unsteadiness.
AND thus we can behold how false, and also the unsteadiness of the hearts of the children of men; yea…how quick to do iniquity, and how slow to do good, are the children of men,…how quick to boast, and do all manner of that which is iniquity; and how slow are they to remember the Lord their God, and to give ear unto his counsels, yea, how slow to walk in wisdom’s paths! (Hel. 12:1-5)