The opposite of faith is not doubt, but unbelief. Thomas doubted, and Peter denied, but both were restored by our Lord. Why? He knew that at the heart of their lives was genuine faith, saving faith, and their times of doubting were momentary.
Judas Iscariot had doubts, as well, but they were based in unbelief. How do we know? His character and conduct. At some point along the way, the one will always produce the other. We act on what we believe. Thomas and Peter persevered. Judas defected. All three acted on what they believed, which seemed to be the same at the time, but Judas' apostatizing revealed his true character.
The beloved apostle John speaks of three characteristics of apostates in I John, chapter 2. He is writing to enlighten and to alarm. He is intensely concerned that the congregations recognize (1) that the apostates' defection reveals their character; (2) that their teachings reveal their unbelief; and (3) that theirs is a deadly serious impact.
In v. 18 he says that "many antichrists have appeared," which characterizes the last hour (in which we are living, as well). His use of the plural distinguishes them from The Antichrist mentioned in Revelation 13:1-5 and 19:20. Their spirit of opposition to the Lord Jesus is the same, though, (straight from the pit), and manifests itself in several ways.
First, their defection. "They went out from us...so that it would be shown that they all are not of us," (v. 19). In the first century, as now, within the visible church there were those who professed Christ, who knew and used the language of faith, who even rose to levels of leadership, but who eventually left, no doubt taking some with them. This confused the remaining ones, and John assures them that such behavior stems from false faith.
Had their faith been genuine they would have endured (Mark 13:13; Hebrews 3:14), manifesting the reality of authentic, saving faith.
Secondly, their doctrine. "Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son. Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also," (vv. 22, 23). Though most likely eloquent in speech, convincing in manner, and authoritative in tone, John calls them liars and heretics because of their denial of sound doctrine. In chapter 4 of this same letter, he adds, "every spirit that does not confess Jesus is not from God; this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world," (v. 3).
Distortion of the nature of the Lord Jesus, according to the whole counsel of God embodied in the Bible, is a crucial indication of apostasy. Denying His deity, downplaying His humanity, ignoring His wrath and judgment, focusing exclusively on His love and compassion, refusing His statements on the reality of hell, rejecting His sovereignty, are but a few of the ways false teachers and deceivers deny the Scriptural presentation of the Lord Jesus Christ. In so doing, John asserts, it's demonstrated that the apostate knows neither the Son nor the Father.
Finally, their deception. The apostle unmasks the motive of the false teachers: "These things I have written to you concerning those who are trying to deceive you," (v. 26). Since we act on what we believe, knowing the truth is of paramount importance. Indeed, the father of lies does all that he can to trivialize the truth, downplay the eternal consequences that are at stake, and keep genuine believers from discerning the seriousness of the situation.
The time has arrived when even believers "will not endure sound doctrine, but wanting to have their ears tickled, will accumulate for themselves teachers in accordance to their desires, and will turn away their ears from the truth and will turn aside to myths," (II Timothy 4:3,4).
May the living Lord of His church awaken sleepy believers to not only be enlightened, but alarmed, grasping the implications of apostasy, and willing to confront the error all about us. He will hold us accountable for our roles as watchmen (Ezekiel 33:7-9).