For the Lord will have mercy on Jacob, and will yet choose Israel, and set them in their own land; and the strangers shall be joined with them, and they shall cleave to the house of Jacob.
While a quotation from Isaiah (and this verse is quoted word for word, unlike say verse 2), this verse manages to encapsulate one of the major messages of the Book of Mormon. Despite misdeeds, trials and tribulations, God has not forgotten Israel, and will have mercy upon them and keep His covenants with them; meanwhile salvation for the Gentiles required becoming part of the House of Israel. While there were exceptions, this was not a common view at the time of the publication of the Book of Mormon; most of Christianity was supercessionist at least in part, believing that the Gentile Church had replaced or was the true continuation of Israel. The Book of Mormon declares the opposite: Israel has not been forgotten, God is about to fulfil his covenants with them in restoring them spiritually and physically, and that the Gentiles need to repent or face the judgment of God.
God’s long suffering, mercy and faithfulness towards a people to whom he has made promises can of course be reassuring to us on a individual scale. Despite the elapse of hundreds of years, God had not forgotten Israel. Likewise, despite our own personal weakness and wanderings, he will not forget us (Isaiah 49:15-16) and “he is faithful that promised” (Hebrews 10:23).