I can appreciate your question here, especially that you didn’t simply isolate one sin but rather equated one sin to all sin. It seems like you’re getting at whether they allowance of a believer for sin under certain circumstances is ok to do, and further more if that has implications for salvation.
First let me commend you on pointing out that sin is sin is sin—no matter what that sin may be. To an infinitely holy and perfect God, all sin is equally distant from His standard of perfection, and everyone of us is guilty of sin, for the Bible tells us that “all have sinned and fallen short of the glory of God.” (Romans 3:23)
Fortunately, it is not us humans who get to allow for sin, decide what constitutes it, nor identify conditions when it is permissible. If we do, we are deceiving ourselves—in fact, that would be determining the standard for good and evil, which was what happened during the fall of man (Genesis 3:5) in the first place. So, you can rest assured that none of us (believers or not) get to play God and determine when something is a sin. That said, are there certain times when a sin could be used by God to bring about good or to work for his glory? Absolutely. But that never changes the nature of the sin in question (I’m thinking of David and Bathsheba here, in 2 Samuel 11). For “we know that for those who love God all things work together for good, for those who are called according to his purpose.” (Romans 8:28)
Some believers during the Holocaust were hiding Jews to protect them in Europe. If the Nazi’s came to their door and inquired if there were any Jews, those individuals could have said no in order to protect the Jews. And some did just that. In God’s eyes, one could ask, wouldn’t that seem like a time where that sin of lying could be excused? Us humans might find it easy to say yes, however, we don’t see allowance for that in scripture. The same would be true for any other sin, including murder (e.g., abortion, to speak to your question specifically). So, individuals who would’ve lied to protect Jews in World War II would be in a position to have sin to confess, just as would someone who, as you put it, makes allowance for abortion under certain conditions. But we know that Christ died for our sins, and this is of first importance that he did (1 Corinthians 15:3) this for all our sins past, present, and future. That means even those sins we commit after being saved. There’s true freedom in that, because it means that we don’t have to worry about jeopardizing our salvation by future sin if we are in Christ, “for by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast” (Ephesians 2:8-9). And so, a believer needs to repent of, and confess, their sins regularly but need not worry that they have fallen out of grace because of their daily sin. How do we know this? 1 John 1:9 “If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.”
So, we can have great confidence in what scripture has to say about the issue of sin’s effect on salvation—that sin doesn’t nullify or void our salvation. Jesus laid this out clearly for Peter and the Apostles when he washed their feet, saying “The one who has bathed does not need to wash, except for his feet, but is completely clean.” (John 13:10)
Jesus is the one who has cleansed us of our sin through His blood on the cross. “So if the Son sets you free, you will be free indeed” (John 8:36). And that means that nothing “will be able to separate us from the love of God in Christ Jesus our Lord” (Romans 8:38-39).
Now, to be clear, no one gets to set allowances for any sin, but should they choose to do so (and subsequently sin as a result of their decisions) they can confess that sin and be forgiven. They do not need to worry about their salvation being lost.
One other thing to consider is this: for those who completely disregard God’s word and commandments, or who do not acknowledge and confess their sin, there is a different issue to be concerned with altogether—and that is whether they are truly saved in the first place. For those who are saved have the Holy Spirit dwelling in them, sanctifying and growing them spiritually to maturity each and every day so that their lives will show marked (but not to be confused with perfect) change after being saved. That’s why Jesus told his disciples that the sign of a true follower of Christ is that they will bear spiritual fruit in their lives, saying: “You did not choose me, but I chose you and appointed you that you should go and bear fruit and that your fruit should abide” (John 15:16). So it could be in question as to whether or not someone has truly received the gift of salvation by faith alone in Christ alone if they continue to live in a constant state of sin, turn a blind eye toward their own sin, or do not acknowledge and repent of the sin in their own life. For we cannot serve two masters (Matthew 6:24), and “Jesus answered them, "Truly, truly, I say to you, everyone who practices sin is a slave to sin” (John 8:34). So, if you continue to serve Satan by being fully of this world and not putting of the old self and putting on the new self (Ephesians 4:22), it’s possible that you have not truly committed your life to Christ and accepted the free gift of His grace.
I hope this is helpful to you and gives you some clarity on an important question. Making allowances for sin under certain considerations would not have Biblical support, and would in no way be any “less” of a sin; however, sin under any circumstances can be confessed by a believer and will be forgiven by God. The only instance the Bible talks about that deviates from this comes in Mark 3:29 when Jesus said ”whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin.” Here Jesus is referring to someone who denies all the work of the Holy Spirit, which leads ultimately to a rejection of Christ, and thus God altogether, which would be impossible for a believer to do, because “no one can say ‘Jesus is Lord’ except in the Holy Spirit.” (1 Corinthians 12:3) Likewise, no one in whom the Holy Spirit swells can deny Christ.
So, I would close by asking you to consider not what effect sin has on salvation, but rather how salvation affects sin. Because in the end, as the apostle Paul wrote to the Christians in Galatia, “I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.” (Galatians 2:20)