The description here is very apt—wolves in sheep's clothing. They appear on the outside to be something they are not. When Jesus uttered this, He was probably thinking of false ministers who would insinuate themselves into the church by appearing to be sheep within the sheepfold.
Jesus uses this terminology in regard to His relationship with the church. He was the Shepherd, and we are His sheep. Here we have wolves (false ministers) who look like sheep, but it is hypocrisy. They only look that way on the outside. He tells us we will know them by their fruits. The fruit that is produced will not necessarily appear quickly. But Christ guarantees that over a period of time the church will be stripped of its true spiritual vitality in terms of the character that will be produced within the flock, making the rise of wolves in sheep's clothing more likely.
What is He saying? The implication is that Jesus is connecting belief with practice. If we believe a certain set of doctrines, we will practice something because of the teaching. A religious creed or the dogma that a group is following will produce a certain kind of conduct by the people. Belief and practice, creed and conduct—Jesus is saying they are vitality connected. In other words, the teachers cannot hide what they are going to produce. Eventually it will come out. Their false philosophies, no matter how attractive they may appear at first sight, will in the long run be exposed for what they really are.
John W. Ritenbaugh
Satan (Part 2)
Once regenerated by the Holy Spirit from the Father, we must continually be led by it, bearing spiritual fruit throughout our lives. If we are producing the fruit of the Spirit, which exhibit a sound mind, we know it is working in us. The Spirit is the mind and essence of the divine nature, and through it God carries out His will. It empowers the mind to comprehend spiritual matters, producing conversion. It gives us the strength, will and faith to overcome our sins.
Martin G. Collins
The Holy Spirit
Jesus does not spell out what "fruits" to look for, although in the Olivet Prophecy, He does link the deceptions of false prophets with the lawlessness and lack of love that abounds at the end time (Matthew 24:11-13). However, the rest of the Bible elucidates God's character and nature, so we already have the tools to evaluate whether a message allegedly coming from God fits with what His Word reveals about Him. God is not double-minded; He will not contradict Himself.
David C. Grabbe
What Is a False Prophet?
Combining the principle that whatever is sown will also be reaped (Galatians 6:7-8) with the tendency towards increase means that no matter what a person sows—unless somebody or something intervenes and interrupts the cycle—more will be produced than was sown.
Living according to Christian standards, or morals and ethics, can be faked for a while. Consider these verses: "Every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit." Christian standards, and morals, and ethics can be faked for a while. But, no matter how guarded a person is, the fruits produced will betray him. That principle that Jesus gave was also given in the Old Testament by Moses, in Numbers 32:23, where Moses said you can "be sure your sin will find you out."
Bitterness, divisiveness, envy, jealousy, lethargy will eventually show. How much of the fruit will be produced cannot be accurately predicted because there are too many variables involved. But because of the principle of increase over what was originally sown—whether good or evil—there is every likelihood that more is going to be produced than was sown.
John W. Ritenbaugh
Every Action Has a Reaction
The tongue utters only what is already in the heart (Matthew 12:34). Acts of sin besides what the tongue commits proceed from the same evil heart (Matthew 15:17-20). Matthew 7:16-18 confirms the impossibility of changing an evil heart, as it can never produce good fruit. God must completely replace it with a wholly different nature for it to produce good fruit! Before being changed, it can produce only evil fruit.
John W. Ritenbaugh
The Offerings of Leviticus (Part Seven): The Sin and Trespass Offerings
Other Forerunner Commentary entries containing Matthew 7:17:
2 Peter 1:10-11