Having brought up his father’s faith in the Son of God, Nephi began an aside that would give more information about that God. Nephi not only testifies, but elaborates on God. Verse 18 echoes Hebrews 13:8, which says “Jesus Christ the same yesterday, and to day, and for ever.”
In verse 19, the language echoes Deuteronomy 4:29, which says “thou shalt seek the Lord thy God, and thou shalt find him.” The language of the mysteries comes from the New Testament, as the word mysteries is not found in the King James Version of the Old Testament.
These are examples of the process of translation, which often pulls language from the Bible, including phrases from the New Testament.
Nevertheless, the concepts that Nephi is explaining are not unusual for his time. In particular, the concept embodied in the phrase “no unclean thing can dwell with God” is an expansion of the cleanliness laws. While other phrases are found in the Bible, that one is not. What is found is the concept of separation for uncleanliness, and here Nephi expands the mortal physical separation into a spiritual separation required by spiritual uncleanness.