1 John 4:1-3
1. Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
1. Dilecti, ne omni spiritui credatis, sed probate spiritus, an ex Deo sint; quia multi pseudoprophetae exierunt in mundum.
2. Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
2. In hoc cognoscite Spiritum Dei; omnis spiritus qui confitetur Jesum Christum in carne venisse, ex Deo est:
3. And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
3. Et omnis spiritus qui non confitetur Jesum Christum in carne venisse, ex Deo non est; et hic est antichristus, de quo audiistis quod venturus sit; et nunc jam in mundo est.
He returns to his former doctrine, which he had touched upon in the second chapter; for many (as it is usual in new things) abused the name of Christ for the purpose of serving their own errors. Some made a half profession of Christ; and when they obtained a place among his friends, they had more opportunity to injure his cause. Satan took occasion to disturb the Church, especially through Christ himself; for he is the stone of offense, against whom all necessarily stumble who keep not on the right way, as shewn to us by God.
But what the Apostle says consists of three parts. He first shews an evil dangerous to the faithful; and therefore he exhorts them to beware. He prescribes how they were to beware, that is, by making a distinction between the spirits; and this is the second part. In the third place, he points out a particular error, the most dangerous to them, he therefore forbids them to hear those who denied that the Son of God appeared in the flesh. We shall now consider each in order.
But though in the passage this reason is added, that many false prophets had gone forth into the world, yet it is convenient to begin with it. The announcement contains a useful admonition; for if Satan had then already seduced many, who under the name of Christ scattered their impostures, similar instances at this day ought not to terrify us. For it is the case perpetually with the Gospel, that Satan attempts to pollute and corrupt its purity by variety of errors. This our age has brought forth some horrible and monstrous sects; and for this reason many stand amazed; and not knowing where to turn, they cast aside every care for religion; for they find no more summary way for extricating themselves from the danger of errors. They thus, indeed, act most foolishly; for by shunning the light of truth, they cast themselves into the darkness of errors. Let, therefore, this fact remain fixed in our minds, that from the time the Gospel began to be preached, false prophets immediately appeared; and the fact will fortify us against such offenses.
The antiquity of errors keeps many, as it were, fast bound, so that they dare not emerge from them. But John points out here all intestine evil which was then in the Church. Now, if there were impostors mixed then with the Apostles and other faithful teachers, what wonder is it, that the doctrine of the Gospel has been long ago suppressed, and that many corruptions have prevailed in the world? There is, then, no reason why antiquity should hinder us to exercise our liberty in distinguishing between truth and falsehood.
1 Believe not every spirit When the Church is disturbed by discords and contentions, many, as it has been said, being frightened, depart from the Gospel. But the Spirit prescribes to us a far different remedy, that is, that the faithful should not receive any doctrine thoughtlessly and without discrimination. We ought, then, to take heed lest, being offended at the variety of opinions, we should discard teachers, and, together with them, the word of God. But this precaution is sufficient, that all are not to be heard indiscriminately.
The word spirit I take metonymically, as signifying him who boasts that he is endowed with the gift of the Spirit to perform his office as a prophet. For as it was not permitted to any one to speak in his own name, nor was credit given to speakers but as far as they were the organs of the Holy Spirit, in order that prophets might have more authority, God honored them with this name, as though he had separated them from mankind in general. Those, then, were called spirits, who, giving only a language to the oracles of the Holy Spirit, in a manner represented him. They brought nothing of their own, nor came they forth in their own name, but the design of this honorable title was, that God's word should not lose the respect due to it through the humble condition of the minister. For God would have his word to be always received from the mouth of man no otherwise than if he himself had appeared from heaven.
Here Satan interposed, and having sent false teachers to adulterate God's word, he gave them also this name, that they might more easily deceive. Thus false prophets have always been wont superciliously and boldly to claim for themselves whatever honor God had bestowed on his own servants. But the Apostle designedly made use of this name, lest they who falsely pretend God's name should deceive us by their masks, as we see at this day; for many are so dazzled by the mere name of a Church, that they prefer, to their eternal ruin, to cleave to the Pope, than to deny him the least part of his authority.
We ought, therefore, to notice this concession: for the Apostle might have said that every sort of men ought not to be believed; but as false teachers claimed the Spirit, so he left them to do so, having at the same time reminded them that their claim was frivolous and nugatory, except they really exhibited what they professed, and that those were foolish who, being astonished at the very sound of so honorable a name, dared not to make any inquiry on the subject.
Try the spirits As all were not true prophets, the Apostle here declares that they ought to have been examined and tried. And he addresses not only the whole Church, but also every one of the faithful.
But it may be asked, whence have we this discernment? They who answer, that the word of God is the rule by which everything that men bring forward ought to be tried, say something, but not the whole. I grant that doctrines ought to be tested by God's word; but except the Spirit of wisdom be present, to have God's word in our hands will avail little or nothing, for its meaning will not appear to us; as, for instance, gold is tried by fire or touchstone, but it can only be done by those who understand the art; for neither the touchstone nor the fire can be of any use to the unskillful. That we may then be fit judges, we must necessarily be endowed with and directed by the Spirit of discernment. But as the Apostle would have commanded this in vain, were there no power of judging supplied, we may with certainty conclude, that the godly shall never be left destitute of the Spirit of wisdom as to what is necessary, provided they ask for him of the Lord. But the Spirit will only thus guide us to a right discrimination, when we render all our thoughts subject to God's word; for it is, as it has been said, like the touchstone, yea, it ought to be deemed most necessary to us; for that alone is true doctrine which is drawn from it.
But here a difficult question arises: If every one has the right and the liberty to judge, nothing can be settled as certain, but on the contrary the whole of religion will be uncertain. To this I answer, that there is a twofold trial of doctrine, private and public. The private trial is that by which every one settles his own faith, when he wholly acquiesces in that doctrine which he knows has come from God; for consciences will never find a safe and tranquil port otherwise than in God. Public trial refers to the common consent and polity of the Church; for as there is danger lest fanatics should rise up, who may presumptuously boast that they are endued with the Spirit of God, it is a necessary remedy, that the faithful meet together and seek a way by which they may agree in a holy and godly manner. But as the old proverb is too true, "So many heads, so many opinions," it is doubtless a singular work of God, when he subdues our perverseness and makes us to think the same thing, and to agree in a holy unity of faith.
But what Papists under this pretense hold, that whatever has been decreed in councils is to be deemed as certain oracles, because the Church has once proved them to be from God, is extremely frivolous. For though it be the ordinary way of seeking consent, to gather a godly and holy council, when controversies may be determined according to God's word; yet God has never bound himself to the decrees of any council. Nor does it necessarily follow, that as soon as a hundred bishops or more meet together in any place, they have duly called on God and inquired at his mouth what is true; nay, nothing is more clear that they have often departed from the pure word of God. Then in this case also the trial which the Apostle prescribes ought to take place, so that the spirits may be proved.
2 Hereby, or by this, know ye He lays down a special mark by which they might more easily distinguish between true and false prophets. Yet he only repeats here what we have met with before, that as Christ is the object at which faith aims, so he is the stone at which all heretics stumble. As long then as we abide in Christ, there is safety; but when we depart from him, faith is lost, and all truth is rendered void. 
But let us consider what this confession includes; for when the Apostle says that Christ came, we hence conclude that he was before with the Father; by which his eternal divinity is proved. By saying that he came in the flesh, he means that by putting on flesh, he became a real man, of the same nature with us, that he might become our brother, except that he was free from every sin and corruption. And lastly, by saying that he came, the cause of his coming must be noticed, for he was not sent by the Father for nothing. Hence on this depend the office and merits of Christ.
As, then, the ancient heretics departed from the faith, in one instance, by denying the divine, and in another by denying the human nature of Christ; so do the Papists at this day: though they confess Christ to be God and man, yet they by no means retain the confession which the Apostle requires, because they rob Christ of his own merit; for where freewill, merits of works, fictitious modes of worship, satisfactions, the advocacy of saints, are set up, how very little remains for Christ!
The Apostle then meant this, that since the knowledge of Christ includes the sum and substance of the doctrine respecting true religion, our eyes ought to be directed to and fixed on that, so that we may not be deceived. And doubtless Christ is the end of the law and the prophets; nor do we learn anything else from the gospel but his power and grace.
3. And this is that spirit of Antichrist The Apostle added this, to render more detestable the impostures which lead us away from Christ. We have already said that the doctrine respecting the kingdom of Antichrist was well known; so that the faithful had been warned as to the future scattering of the Church, in order that they might exercise vigilance. Justly then did they dread the name as something base and ominous. The Apostle says now, that all those who depreciated Christ were members of that kingdom.
And he says that the spirit of antichrist would come, and that it was already in the world, but in a different sense. He means that it was already in the world, because it carried on in secret its iniquity. As, however, the truth of God had not as yet been subverted by false and spurious dogmas, as superstition had not as yet prevailed in corrupting the worship of God, as the world had not as yet perfidiously departed from Christ, as tyranny, opposed to the kingdom of Christ, had not as yet openly exalted itself, he therefore says, that it would come.
 It appears that by "spirit" throughout this passage, we are to understand a teacher claiming, rightly or falsely, to be influenced by God's Spirit. Nor would it be improper, but suitable to the context, to consider the spirit of God in this verse as meaning a teacher guided by God. The meaning of the passage might be thus expressed, -- 2. "By this know ye the teacher of God; every teacher who confesses Jesus Christ as having come in the flesh, is from God; and 3. every teacher who does not confess Jesus Christ as having come in the flesh, is not from God; and this is the teacher of Antichrist, (or, the Anti Christian teacher,) of whom ye have heard that he is coming, and he is now already in the world." -- Ed