Legalism overthrows the gospel, since it trusts, at least partly, in one's own self-righteousness and not Christ alone for salvation. It erroneously believes that God is somehow satisfied with our pathetic attempts to maintain our right standing with Him by being moral. (Gal 3:3; Phil 3:3) But this is to usurp Christ's office as Savior. Perhaps the worst offenders of this within the visible church are those who believe we can lose our salvation, thinking that Christ's work is incomplete or insufficient and that we must somehow make up for it with our own enduring faith or merits or else risk losing our status as His adopted children. But Christ "is able to save to the uttermost those who draw near to God through him, since he always lives to make intercession for them." (Heb 7:25) "For by a single offering he has perfected for all time those who are being sanctified." (Heb 10:14)
Those persons in the visible church who say that a regenerate believer can lose their salvation (or forfeit their faith) may indeed affirm that Jesus' atonement paid for past sins, but since they believe our post-regeneration sin can disqualify us, it would mean that their future justification is dependent, in whole or in part, in their own ability to persevere to the end. As such they are not trusting in Jesus Christ alone for their redemption but partly in their own efforts and ability. The Bible indeed teaches that we must persevere to the end but it also teaches that those who are born again will not, indeed CANNOT life a lifestyle of sin because God's seed dwells in them ... and since Jesus always lives to intercede for believers, He will preserve them to the end (1 John 3:9; John 6:37-39).