Matthew 7:15-20 does NOT mean that you will know a teaching or a teacher is true or false based upon the good or bad RESULTS of the teaching (i.e. good works, good feelings, fruit of the Spirit, etc).
Jesus explicitly explained what He meant in Matthew 12:33-37,
"Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.
O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh.
A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
For by thy words thou shalt be justified, and by thy words thou shalt be condemned."
And also in the parallel passage in Luke 6:43-45,
"For a good tree bringeth not forth corrupt fruit; neither doth a corrupt tree bring forth good fruit.
For every tree is known by his own fruit. For of thorns men do not gather figs, nor of a bramble bush gather they grapes.
A good man out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil: for of the abundance of the heart his mouth speaketh."
In other words, Jesus meant that we will know a teaching or a teacher is true or false based upon the TEACHING, not based upon the results of the teaching. The question is: "what do they teach?" Not, "what is the result of what they teach?"
This is in keeping with the Old Testament test of a false prophet,
"If there arise among you a prophet, or a dreamer of dreams, and giveth thee a sign or a wonder,
And the sign or the wonder come to pass, whereof he spake unto thee, saying, Let us go after other gods, which thou hast not known, and let us serve them;
Thou shalt not hearken unto the words of that prophet, or that dreamer of dreams: for the LORD your God proveth you, to know whether ye love the LORD your God with all your heart and with all your soul.
Ye shall walk after the LORD your God, and fear him, and keep his commandments, and obey his voice, and ye shall serve him, and cleave unto him.
And that prophet, or that dreamer of dreams, shall be put to death; because he hath spoken to turn you away from the LORD your God, which brought you out of the land of Egypt, and redeemed you out of the house of bondage, to thrust thee out of the way which the LORD thy God commanded thee to walk in. So shalt thou put the evil away from the midst of thee." (Deuteronomy 13:1-5)
"To the law and to the testimony: if they speak not according to this word, it is because there is no light in them. (Isaiah 8:20)
It is high time for you to start listening to Jesus and to test a prophet by the content of his teaching rather than by the results of his teaching.
