"By their works?"

I am on a quest to read through the Bible in 6 months. Usually, I read from Genesis to Revelation, in book order. This time, I am reading a selection from the First Testament (minus Psalms and Proverbs) a selection from the New Testament, a Psalm and and a few verses from Proverbs every day. The juxtaposition of thoughts is sometimes startling, either because they seem so different, or, as today, because the readings, especially the First and New Testament readings, seem so closely in harmony.
In Deuteronomy 13, Moses tells the Children of Israel to be careful about people who come as prophets. Verses 1 and 2 give the "fruit" test for a prophet - they predict the future, and their prediction is accurate, or they perform some sign or wonder. In other words, the prophets seem to be acting by the power of God, because they are correct in their foreseeing or demonstration of power. Verse 3 gives the warning - unless their teaching is in line with that of Moses, their demonstrations of power are to be ignored.
In Matthew 7 (parallel to Luke where I was reading in the NT), Jesus warns about "servants" who have done miraculous things. "Lord, Lord, in your name we did these amazing things... even raising the dead..." (my paraphrase of Matt 7:22) But just before this section, which is a lot like Moses in Deuteronomy, Jesus tells us that we can know people by their "fruit" - the things they do.
Jesus and Moses both seem to be saying that demonstrations of power are not the justification of a person's life. Just because someone can do amazing things is not proof that they are living a righteous or godly life. Demonstrations of power are also not sufficient to prove that what someone says is correct, or godly, or proper. For the people of Israel, they were to follow God even if a powerful person tried to convince them otherwise. For followers of Jesus, acts of power are not sufficient to guarantee eternal life.
For believers today, the fact that God is using someone to bring about His purpose does not mean that the person is blessed by God. In all cases, we must weigh our own actions by the commands of God; we must carefully consider the teachings of others, and ensure they are aligned with the Word of God. I am not responsible for you, though I have a responsibility to you. I am responsible for my own actions, attitudes, and behaviours before God,just as you are responsible for yours. Though we walk this Christian life together, we stand or fall alone before the holy God.