Not To Be Confused With
Careful Reformed theology states this as perseverance (the saved persevere BECAUSE God keeps them, and a faith that dies was never saving). The caricature — a past decision guarantees heaven regardless of any ongoing faith or life — is rejected by Reformed and non-Reformed alike.
At its best, 'eternal security' is the comfort of John 10:28 and Romans 8:38-39 — nothing can separate the believer from God's love. The danger is a cheapened version severed from ongoing faith and perseverance. Arminian and Catholic traditions hold that real believers may apostatize.