In His Word, God says He never changes. His character remains the same. They lived by the Law with its morals.
With that in mind, the Deuteronomy passage says that anyone claiming to be a prophet, but contradicting God's existing Word (eg commands), is a liar. Most people falsely claiming to be prophets have poor character, be ego/money centered, people pleasers, often contradict the Word, cherry-pick what serves them, and/or fail in their predictions or miracles (see Deut. 18). The Deut. 13 test is even easier today since we have the whole Word, including Jesus' and Apostles' ministry, to test them with.
If you give to others, you will be given a full amount in return. It will be packed down, shaken together, and spilling over into your lap. The way you treat others is the way you will be treated. (Luke 6:38)
You and I know this verse is talking about forgiveness, but the Bible contains very many verses that can, out of context, be used to justify a great many evils.
Back to your original point:
> Followers of Jesus Christ are commanded and equipped to test prophets rather than blindly believing them.
Recent history shows us that followers of Jesus also make spectacularly good marks for anyone with a good sales pitch and some memorised lines of New Testament teaching.
With that in mind, the Deuteronomy passage says that anyone claiming to be a prophet, but contradicting God's existing Word (eg commands), is a liar. Most people falsely claiming to be prophets have poor character, be ego/money centered, people pleasers, often contradict the Word, cherry-pick what serves them, and/or fail in their predictions or miracles (see Deut. 18). The Deut. 13 test is even easier today since we have the whole Word, including Jesus' and Apostles' ministry, to test them with.