I have a nephew wrapped up with "Psalm 119 Ministries," which is a cult, following the Torah and observing the Sabbath on Saturday and "obeying" other practices and commands of Moses. They don't observe Christmas or Easter because they are "pagan" celebrations. Also the "Trinity" is out the window! All church denominations are "wrong" because they worship "doctrines of men." Being in strict "obedience" to the Torah is paramount! They acknowledge Jesus as the Messiah and Redeemer, but that is always in the background. I have used St. Paul's epistles, especially Galatians, but he always says it is so easy to twist Paul's words. Is there anything you could encourage me with that I might not have tried?
The organization you referenced does speak of the Trinity, but they promote significant errors in their Statement of Faith. They speak of multiple judgments before God and a literal 1,000-year reign of Christ on earth. I found no mention of the sacraments in their Statement of Faith. Their misunderstanding of Old Testament ceremonial laws is evident in this statement: “All Scripture (Word) is still true and nothing has been abolished.” The ceremonial laws have been abolished. The tearing of the temple curtain on Good Friday illustrated that (Matthew 27:51).
The book of Galatians was a good part of the Bible to share with your nephew. Nothing is being twisted when you point to clear words like these: “Formerly, when you did not know God, you were slaves to those who by nature are not gods. But now that you know God—or rather are known by God—how is it that you are turning back to those weak and miserable forces? Do you wish to be enslaved by them all over again? You are observing special days and months and seasons and years! I fear for you, that somehow I have wasted my efforts on you” (4:8-11).
Christian freedom is one of the great themes of Galatians. The book of Colossians also highlights that theme of Christian freedom (cf. especially 2:16-17).
What can you do for your nephew? First, recognize that only God can change a person’s heart and mind. He does that through his word, so keep sharing appropriate Scripture passages with your nephew. The WELS web site has numerous devotions available. Perhaps you could share ones that are especially appropriate to his circumstances.
In addition, keep praying that God leads your nephew to see and confess the truths of his word. Prayer is powerful and effective (James 5:16).
Also, as you engage your nephew in conversation about his faith, ask questions. You are not looking to back him into a corner; you are seeking to understand more what he believes and where he thinks he has scriptural support for his beliefs. His answers can help determine direction for your witnessing—either at that time or in the future. God bless your efforts to testify to the truths of his word!