There is no break in the thought where the chapter is divided, but John is proceeding to offset what he has just said about the Holy Spirit by warning us to try the spirits, so that we may know whether they are of God.  All of the tests of assurance he has brought to our attention through the first 3 chapters are tests whereby we may discern that we have the true Spirit of God.  The Holy Spirit will testify of Christ and glorify Him, He will work in us love to the brethren, as well as love and obedience towards the law of God.  If the spirit that motivates us and directs us is doing these things, then we may be comfortably certain that it is the Holy Spirit.

However, we are not simply to assume that every spirit (and spirit here can be synonymous with doctrine, for false doctrines are also propagated by spirits) that says something positive about Jesus is of God.   It is well to recollect from John’s gospel that there were those who professed some faith in Christ, and yet He did not commit Himself to them, because He knew all men, and discerned that that which was in their hearts was not saving faith.  Christ is the great and the ultimate tester of spirits.  Nevertheless, we are also commanded to weigh the spirits, and discern those which be of God as opposed to those which are of the Devil.   The great scale in which we weigh the spirits is word of God.  Any doctrine, whoever it is propagated by, that does not measure up to the infallible standard of sacred Scripture, is immediately to be rejected.  When it is an assault made upon some cardinal doctrine, such as the deity of Christ, His blood atonement, His second coming, to name but a few, the promoter of those lies should be denounced as a heretic and sent packing.  We must ever vigilantly guard ourselves and our churches against these impostors, because, as John here warns us, there are many false prophets gone out into the world.  They are smooth and clever, and if it were possible they would deceive even God’s very elect.