Sometimes we have the problem dealing with people and we often fall into the trap if we do not know how to discern well. Remember, our Lord Jesus said; “They come to you in sheep’s clothing, but inwardly they are ferocious wolves” Matthew 7:15. To know them by their fruits means that you can recognize person’s real identity by looking at the evidence shown by that person’s life. You can tell what they are by what they do or in another way of saying it, one can detect them by the way they act. Other passage in Matthew clearly expresses the same concept in verse 12:33 which Jesus says; “”Make a tree good and its fruit will be good or make a tree bad and its fruit will be bad, for a tree is recognized by its fruit.”
Brian A. Yeager of “Words of Truth Weekly” said; “sometimes, we don’t want to see the wrong in people. Sometimes, we close our eyes to things we should be seeing. Sometimes it is our family members we cannot really see for who they are (cf. Matthew 12:46-50). Jacob certainly struggled with seeing what his children did with their brother Joseph though there had been jealousy for some time in the family (Genesis 37). Sometimes, we get caught in emotions and do not think clearly. For example, though Delilah had betrayed Samson, he did not heed the clear warning signs before him (Judges 16:6-20). Sometimes, it is someone who convinces us that they are on God’s side and because of this we cannot see the warning signs. The Old Prophet was able to deceive the younger prophet for this reason (I Kings 13:11ff).” (http://www.wordsoftruth.net/wotvol9/wotbulletin09062009.html). Christians should be aware that our enemy is deceptive. Satan plays like that and has been a liar from the beginning. He comes in disguise as a serpent that was proven in Genesis 3 as it says in verse 1 “Now the serpent was more crafty than any of the wild animals the LORD God had made. He said to the woman, “Did God really say, ‘You must not eat from any tree in the garden’?” This malevolent creature is so cunning and knows how to deceive God’s people if they are not strong.
Those whose fruits are not in accordance with God’s will or choose not to remain with Christ are to be “thrown away”. They are trash; unfit for further use. Christians have nothing to do with them. Wayne Jackson says; “six times in John 15:1-7 the Lord used the expression “in me.” The term is employed of those who are in “union” with Christ. It initially referred to those “disciples” to whom he was giving instruction on that occasion; later, though, the application would be to those who have become “disciples” in a more technical sense (Matthew 28:19), and thus have entered into the “in Christ” relationship by means of obedience to the gospel (Romans 6:3-4; Galatians 3:26-27). These became known formally as “Christians” (Acts 11:26; 26:28; 1 Peter 4:16).” (https://www.christiancourier.com/articles/1074-allegory-of-the-vine-and-the-branches-the).Their fruits show they are true followers of Christ yet those who do not show any fruit and are dead branches will cut down and be thrown to the fire.