When eternally secure believers sin, they cannot lose eternal life, but they can lose their eternal rewards. Not many critics of eternal security have a doctrine of eternal rewards and therefore have trouble interpreting Biblical warnings about losing those rewards. Instead, they misinterpret those passages as references to losing salvation.
For example, in Jesus’ letters to the seven churches, the Lord gives examples of kingdom privileges that are promised to “he who overcomes.” The verb is nikaō. Bob Wilkin explains, “In the New Testament an overcomer is a believer who overcomes the world, the flesh, and the devil by persevering in faith and good works until the end of his life (whether by Rapture or death)” (Wilkin, The Ten Most Misunderstood Words, p. 179). Likewise, Tony Evans says “it means to be victorious in the midst of, over, in, or through whatever circumstances are illegitimately holding a believer hostage” (Tony Evans Bible Commentary, p. 1398).
Everyone who overcomes will receive special privileges from the Lord, as Jesus promised to each of the churches.
For example, to the Ephesians, Jesus promised:
“He who has an ear, let him hear what the Spirit says to the churches. To him who overcomes I will give to eat from the tree of life, which is in the midst of the Paradise of God” (Rev 2:7).
To the church in Smyrna:
“He who overcomes shall not be hurt by the second death” (Rev 2:11).
To the church in Pergamos:
“He who has an ear, let him hear what the Spirit says to the churches. To him who overcomes I will give some of the hidden manna to eat. And I will give him a white stone, and on the stone a new name written which no one knows except him who receives it” (Rev 2:17).
To the church in Thyatira:
“And he who overcomes, and keeps My works until the end, to him I will give power over the nations” (Rev 2:26).
To the church in Sardis:
“He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life; but I will confess his name before My Father and before His angels” (Rev 3:5).
To the church in Philadelphia:
“He who overcomes, I will make him a pillar in the temple of My God, and he shall go out no more. I will write on him the name of My God and the name of the city of My God, the New Jerusalem, which comes down out of heaven from My God. And I will write on him My new name” (Rev 3:12).
To the church of the Laodiceans:
“To him who overcomes I will grant to sit with Me on My throne, as I also overcame and sat down with My Father on His throne” (Rev 3:21).
I don’t fully comprehend what those privileges mean, but I can see they are all eternal rewards. And the implication is if you fail to overcome—i.e., if you persist in carnality and spiritual immaturity—you will lose out on those kingdom privileges.
That’s another negative consequence of sin.
Proponents of eternal security insist that everlasting life is unconditionally guaranteed from when you first believe in Jesus. From then on, nothing you do, say, or believe can change the fact that you will spend eternity with God.
However, that does not mean sin has no repercussions for the believer. The Free Grace view accounts for the consequences. And as Revelation 2-3 show, while you cannot lose everlasting life, you can lose out on the kingdom’s sweeter privileges, such as eating fruit from the tree of life (cf. Rev 22:1-2).