Beware of false prophets, which come to you in sheep's
clothing, but inwardly they are ravening wolves. Ye shall
know them by their fruits. Do men gather grapes of thorns, or figs of
thistles? Even so every good tree bringeth forth good
fruit; but a corrupt tree bringeth forth evil fruit. A
good tree cannot bring forth evil fruit, neither can a corrupt tree
bring forth good fruit. Every tree that bringeth not
forth good fruit is hewn down, and cast into the fire.
Wherefore by their fruits ye shall know them. Not every
one that saith unto me, Lord, Lord, shall enter into the kingdom of
heaven; but he that doeth the will of my Father which is in heaven.
From "The Apostolic Fathers with Justin Martyr and Irenaeus: Chapter II. – Cautions against false teachers":
"Every one that teaches anything beyond what is commanded, though he be [deemed] worthy of credit, though he be in the habit of fasting, though he live in continence, though he work miracles, though he have the gift of prophecy, let him be in thy sight as a wolf in sheep's clothing, labouring for the destruction of the sheep. If any one denies the cross, and is ashamed of the passion, let him be to thee as the adversary himself. 'Though he gives all his goods to feed the poor, though he remove mountains, though he give his body to be burned, let him be regarded by thee as abominable. If any one makes light of the law or the prophets, which Christ fulfilled at His coming, let him be to thee as antichrist. If anyone says that the Lord is a mere man, he is a Jew, a murderer of Christ."
St. Gregory Palamas, in his Discourse on the feast of the Entry of the More Pure Lady Theotokos into the Holy of Holies applied this truth of the tree bearing good fruit to the Mother of God…
"If a tree is known by its fruit, and a good tree bears good fruit, then is not the Mother of Goodness Itself, She who bore the Eternal Beauty, incomparably more excellent than every good, whether in this world or the world above? Therefore, the coeternal and identical image of Goodness, Pre-eternal, transcending all being, He Who is the pre-existing and good Word of the Father, moved by His unutterable love for mankind and compassion for us, put on our image, that He might reclaim for Himself our nature…"
Clement of Alexandria discusses v21:
"Let us not merely call him Lord, for that will not save us. For he says,
“Not everyone who says to me, Lord, Lord, will be saved, but he who
does what is right” (Matt. 7:21). Thus, brothers, let us acknowledge him
by our actions, by loving one another, by refraining from adultery,
backbiting, and jealousy, and by being self-controlled, compassionate,
kind. We ought to have sympathy for one another and not to be
avaricious. Let us acknowledge him by acting in this way and not by
doing the opposite. We ought not to have greater fear of men than of
God. That is why, if you act in this way, the Lord said, “If you are
gathered with me in my bosom and do not keep my commands, I will cast
you out and will say to you: ‘Depart from me. I do not know whence you
come, you workers of iniquity.’”