Salvation is secure for everyone who continues trusting and abiding in Christ, and nothing outside a person can snatch them from His hand. Scripture also warns that willful, persistent turning away from Christ — not ordinary struggle or weakness — can result in someone abandoning the faith they once professed.
Romans 8 promises that nothing in all creation can separate believers from the love of God, and Jesus said no one can snatch His sheep out of His hand — powerful assurance for anyone who is trusting Him today. At the same time, Jesus described Himself as the vine and believers as branches, warning that a branch that does not abide in Him withers and is removed, and Hebrews and 2 Peter both warn soberly about those who deliberately, knowingly turn back from a genuine relationship with Christ. The consistent picture is not that salvation is earned or that momentary sin or doubt cancels it, but that salvation is kept secure through an ongoing, living faith relationship with Christ; security is real and trustworthy for all who continue to abide in Him by grace, while Scripture takes seriously the danger of deliberately, finally rejecting Him.
Salvation is entirely by grace through faith in Christ. His atoning sacrifice, heavenly ministry, and intercession are the foundation of assurance; obedience, Sabbath faithfulness, and loving service are the fruit of a redeemed life, never a way to purchase acceptance with God.