In the 16th century a man named Martin Luther stood up against a great body of men who controlled not only the church but the world! That body was the Roman Catholic Church. Many people do not understand the man, nor do they understand his concern or his purpose. There are tons of mis-information available. What is important to know is not who he was but why he did what he did.
The primary issue for Luther was not being right when everyone else was wrong. Indeed, the primary issue was not even the various problems within the Roman Catholic heirarchy. The primary issue that sparked what has come to known as the Protestant Reformation was how a sinner is made right in God's sight, what the Bible calls justification.
Rome has taught, and still teaches, that God will never declare a person just until that person actually, under divine scrutiny, is found to be just. The process of becoming just involves works; i.e. baptism, penance, etc.
I know that this subject has been addressed ad nauseum. Still, we have to ask some very down to earth questions;
If our actual justification depends on us, to any degree, how can we ever be sure that we are accepted of God? In Catholic theology, claiming such assurance is considered a sin (presumption). All that Catholics have to look forward to, after death, is a fiery purgatory that will, in a sense, burn their sins off. Considering the fact that Jesus was made incarnate to definitely save His people from their sins (Matthew 1:21), and considering that, on the cross, the place of atonement for sin, He declared His work finished (John 19:30), does it seem reasonable that God would then leave those who believe on Christ, for eternal life, in suspense? Is 'eternal' life also 'iffy' life? Paul was certain (Philippians 1:6; 2 Timothy 1:12). Beside this, how can there be the peace and joy that the Holy Spirit provides us with (Galatians 5:22) if we are in uncertainty that God accepts us and loves us in Christ (Ephesians 1:6)?
Does it not make sense that God would surely have us to know, for certain, that we are right in His sight in order that we may be confident and bold in denying self, taking up the cross and living for His glory in this evil world? Is it presumptuous to believe that Christ is the perfect and only acceptable sacrifice for our sins and that He who passed to the right hand of God now lives to make intercession for His people (Hebrews 7:25)? Is it a sin to trust in the words of Christ to the effect that all that the Father gives Him will come to Him, that He will lose none, but raise them up at the last day (John 6:37-39)?
Read Romans 8:29-30; Whom He did foreknow (covenant with in advance) He also did predestinate (destine in advance) to be conformed to the image of His Son. These He called and JUSTIFIED and glorified!" Paul jumps from justification, not to sanctification, even though that is important, but to glorifiction or life in eternal glory. Why? Because justification is not a process but an eternally done deal.
When we are convicted of our sin, when we truly repent, when our faith is in the Lord Jesus Christ as our Savior, when we refuse to have any confidence in the flesh or the works of the flesh (Philippians 3:3), when we are truly new creatures in Christ (2 Corinthians 5:17), when we are led by the Spirit (Romans 8:14), we can be sure that we are right in God's sight. If we rely on God's grace thorugh faith + anything of ourselves we can only be in terrible suspense.