In Rev 2:5, we read the following warning to the church at Ephesus that, unless they repent, they will lose their (eschatological) salvation:
Remember therefore from whence thou art fallen, and repent, and do the first works; or else I will come unto thee quickly, and will remove thy candlestick out of his place, except thou repent.
Commenting on this verse, Brian K. Blount writes:
Christ’s counsel could not be clearer. Using the same command he will again issue at 3:3, he orders the Ephesians to remember what they were doing when they demonstrated their first love. Here and at 3:3, repentance is preceded by and no doubt based upon this remembrance. Only after the Ephesians remember both the witnessing and the loving behavior that stood them in right relationship with their fellow church members, and therefore with Christ, can they turn back to it. At 2:16, the command to repent occurs without being introduced by the exhortation to remember, but there it is followed up, as are the repent commands here and at 3:3, with the threat of judgment. That judgment is expressed through the imagery of the lampstands that Christ used to open his remarks in v. 1. The one who walks in the midst of the lampstands and cares for them will remove this one, this Ephesian church, from its place with the others, and thus its place of eschatological relationship with God and Christ, unless the people whom it represents repent. Repentance is a key theme throughout Revelation (see 1:7; 8:7-13; 9:20-21; 11:1, 13; 15:4; 16:9, 11; 22:11); it is a response to God’s forgiving love, the kind of love the Ephesians no longer demonstrate toward one another. (Brian K. Blount, Revelation: A Commentary [New Testament Library; Louisville, Ky.: Westminster John Knox Press, 2009], 51, italics in original, emphasis in bold added)
Here, we have an explicit teaching that: (1) even after conversion, Christians are to engage in repentance and (2) that the book of Revelation teaches that, with the commission of certain heinous sins, one can lose their salvation.