Ephesians 2:8-9 says "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast." (KJV)
This doctrine was established by Protestant Reformer Martin Luther, who considered it "The article with and by which the church stands."
See the following articles by supporters and detractors of the doctrine of sola fide:
Sola Fide (scborromeo.org) Many Bible-believing Protestants are erroneously convinced that the Catholic Church teaches that salvation is by works, that is, one can "earn" or "merit" salvation on his own. Salvation is the unmerited gift of God, and is retained by doing good; but it can be forfeited by doing evil or by simply refusing or neglecting to do good.
9 Things You Should Know About the Council of Trent Joe Carter (thegospelcoalition.org) The Council rejected Protestant doctrines one by one.
Council of Trent: Canons on Justification, Matt Slick (carm.org) The canons contradict scripture.
Once Saved Always Saved - Fact or Fiction? (preparingforeternity.com)
Unconditional Eternal Security: "Salvation in Christ cannot be lost, no matter what one does, because salvation is not dependent on works, but faith in Christ alone. 'As it is written, The just shall live by faith alone.' 'For we hold that a man is justified by faith alone apart from works of the law.' " (Martin Luther's reading of Romans 1:17,3:28.) The author says the doctrine of "once saved, always saved" is a fraud.
Luther Added The Word "Alone" to Romans 3:28? (beggersallreformation.blogspot.com) He was not alone in doing so.
The Aquila Report - Justification by Faith Alone: Martin Luther and Romans 1:17. Justification is a righteousness that God gives freely to those who believe. (theaquilareport.com) The gift of righteousness bestowed on the Christian is a legal judgment, not a spiritual or ontological change
Lingonier Ministries: Justification by Faith Alone: Martin Luther and Romans 1:17
R. C. Sproul describes the moment of awakening Martin Luther had as he read Romans 1:17 (video embedded) (lingonier.org)
|“|| "Be a sinner, and let your sins be strong (sin boldly), but let your trust in Christ be stronger...No sin can separate us from Him, even if we were to kill or commit adultery thousands of times each day."
—(Martin Luther: Letter to Melanchthon)
Total and Complete Forgiveness (josephprince.org) "But you don't confess your sins to God in order to be forgiven. You already have total and complete forgiveness because of the blood of Jesus."
Pastor Joseph Prince in his answer boldly asserts with complete confidence a scripture-based argument that all sins, past, present, and future are already forgiven, and there is no need to confess any sin committed. Salvation in Christ cannot be lost, no matter how heinous and vile the sin might be. All of your sins are already forgiven. God will never condemn you.
However, this position is seen by others as contradicting the Bible's conditional-forgiveness teaching in 1 John 1:9:
"IF we confess our sins - [ then ] he will forgive our sins and cleanse us from all unrighteousness". (John does not say, "We need not confess our sins, for he has already forgiven all of our sins and cleansed us from all unrighteousness") John is not addressing his letter to unbelievers he is seeking to convert, urging them to confess their sins and receive Christ as their Savior once and for all, but to Christians, "fathers, young men, children", who already believe and have hope in him.
See also James 5:14-16 and John 20:21-23
The Truth About Martin Luther: And Why So Few Read His Works (jesus-is-savior.com)
Citations of Martin Luther's "licentious blasphemy" in his own words, from his own works, in context.
Catholic Bible 101: Luther's Revolution (catholicbible101.com)
Luther opened the door to licentious behavior.
Christians Shall Be Rewarded According to Their Works Trumpet Ministries, Inc. (wor.org)
A scripture-based argument that those who have been saved and afterward act wickedly will be irrevocably condemned to hell, unless they repent.
Let God Be True!: Once Saved, Always Saved. One heresy leads to another heresy—requiring two lies! The lie of decisional regeneration needs the lie of guaranteed eternal life to comfort the "saved" that continue living in sin. (audio play option included) (letgodbetrue.com/sermons)
Into the Light Ministries: The Truth About Unconditional Eternal Security (intothelight.org) The author says it is a foul doctrine.
Catholic priest preaches on sin and salvation (youtube.com) Faith alone cannot save you.
Purity of Heart, Pope John Paul II, General Audience of 10 December (ewtn.com) Works of compassion out of a pure heart are necessary.
The Fallacy of Salvation and Justification by Faith Alone (dream-prophecy.blogspot)
Argument from scripture and the writings of the 19th century Swedish philosopher and visionary mystic Emanuel Swedenborg founder of The New Church (New Jerusalem Church) (see Paul's warning: Colossians 2:18-19 with commentaries on 2:18).