Notice that even Jesus’ “thief in the night” analogy is contained within the context of His revealing many signs whereby His disciples would not be caught off guard by His return. So the “thief in the night” analogy cannot be rightly used to prove that none should expect to have any idea of when Jesus will return.
Both Paul and Peter used Jesus’ “thief in the night” analogy when they were writing about “the day of the Lord” (see 1 Thes. 5:2-4, 2 Pet. 3:10). They believed the analogy had application to Jesus’ wrathful return at or near the end of the seven-year Tribulation. Interestingly, however, Paul told his readers, “But you, brethren, are not in darkness, that the day should overtake you life a thief” (1 Thes. 5:4). He rightly interpreted Jesus’ analogy, realizing that those who were alert to the signs and obediently following Jesus were not in the darkness, so that Christ’s coming would not catch them by surprise at all. For them, Jesus would not come like a thief in the night. Only those in darkness would be surprised, which is exactly what Jesus taught. (See also Jesus’ use of the “thief in the night” phrase in Rev. 3:3 and in 16:15, where He uses it in reference to His coming at the battle of Armageddon).
From this point on in the Olivet Discourse, Jesus repeatedly admonished His disciples to be ready for His return. At the same time, He also told them how they could be ready, as He recited the parables of the unfaithful slave, the ten virgins, and the talents, and then foretold the judgment of the sheep and goats (all worth reading). In almost every case, He warned them that hell awaited those who were not ready for His return (see Matt. 24:50-51; 25:30, 41-46.) The way to be ready is to be found doing God’s will when He returns.
 Clearly, for Jesus to warn His closest disciples of not being ready at His return, there existed the possibility of their not being ready. If He warned them of the penalty of eternal punishment for not being ready because of sin, then it was possible for them to forfeit their salvation because of sin. How this should speak to us of the importance of holiness, and the folly of those who say that it is impossible for believers to forfeit their salvation.