Matthew Henry’s Concise Commentary
4:1-6 Christians who are well acquainted with the Scriptures, may, in humble dependence on Divine teaching, discern those who set forth doctrines according to the apostles, and those who contradict them. The sum of revealed religion is in the doctrine concerning Christ, his person and office. The false teachers spake of the world according to its maxims and tastes, so as not to offend carnal men. The world approved them, they made rapid progress, and had many followers such as themselves; the world will love its own, and its own will love it. The true doctrine as to the Saviour’s person, as leading men from the world to God, is a mark of the spirit of truth in opposition to the spirit of error. The more pure and holy any doctrine is, the more likely to be of God; nor can we by any other rules try the spirits whether they are of God or not. And what wonder is it, that people of a worldly spirit should cleave to those who are like themselves, and suit their schemes and discourses to their corrupt taste?
Gill’s Exposition of the Entire Bible
…but try the spirits whether they are of God; not by human reason, especially as carnal and unsanctified; for though the doctrines of the Gospel are not contrary to true reason, they are above it, and not to be judged of by it, and are disapproved of and rejected by carnal reason; but by the word of God, which is the standard of all doctrine; and whatever agrees with that is to be received, and what does not should be rejected. And so to do is very commendable, as appears from the instance of the Beraeans, who on this account are said to be more noble than those of Thessalonica, Acts 17:11; and from the commendation of the church at Ephesus, Revelation 2:2. And this is what every believer, every private Christian should do; to them it belongs to read and search the Scriptures, and prove all things, and judge for themselves of the truth of doctrine; and to such a probation or trial of the spirits, spiritual light, knowledge, judgment, sense, experience, and divine guidance are necessary, which should be asked of God, and an increase thereof; and all such diligent searchers, and humble inquirers, are capable of making judgment of persons and doctrines, whether they are from the Spirit of God or not, for the Spirit of God never speaks contrary to his word…
1 John 4:1
Phillip explains the Scriptures to the Ethiopian Eunuch. 
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