Because of what the bible says about Christ paying for our sins at the cross, and all humans are already born fallen, and committing ANY sin EVER keeps us from being able to earn our way into heaven, I DO believe we are saved by grace through faith in Jesus Christ. I do realize that some Christians debate that once saved, they wonder if one can lose it or not. I tend to think one cannot lose their salvation for ANYTHING, but I understand the debate, even based on certain interpretations of certain passages in the bible. Even though I think Once Saved, Always Saved is OBJECTIVELY true, I do NOT believe you need to believe THAT doctrine in order to truly GET SAVED. I do believe salvation is obtained by faith in Jesus Christ, which in Greek means to "trust in, to adhere to, to rely upon." If you have that kind of faith in Jesus, your spirit is regenerated, you receive the indwelling Holy Spirit, and you are TRULY SAVED.
Part of the debate on losing salvation is either arguing from free will that one can reject Christ at a later date after first having faith in Him, or some really serious sin that kills your faith. I did read in some Catholic literature a long time ago that mortal sin "causes us to DESERVE hell." I STRONGLY disagree with that wording, if one could lose their salvation by some serious sin, it is because it kills your faith, NOT because you offended God to where He revokes your salvation because the sin is too big. ALL SIN causes us to deserve hell. ALL SIN, big OR small, therefore to get into heaven we MUST have God's grace covering us. YOU COULD earn salvation if YOU lived a perfect life and NEVER EVER commit even one SMALL sin, but since ALL have sinned other than infants and small children, this isn't going to cut it. There are some bible verses that indicate that once one has faith in Jesus they have passed from death into life, and that they will never face judgement. This argues for once saved always saved, and also that "warning verses" on certain sins causing people to not inherit eternal life do not apply to believers, but unbelievers showing that without Christ, none of them will be saved because of committing the sins on those lists. In Hebrews it says that faith is COUNTED as righteousness, it is credited to people with faith because their sin makes them imperfect. All the great heroes of the bible committed sins, some great sins, but all of them trusted in the Lord as well, so God's grace covered their sins.