First off, let me began by saying that I
do believe in the doctrine of
sola fide, but I
do not believe in the teaching known as “once saved always saved” or more formally as “eternal security”.
Hitler was baptized and received Holy communion. This means he accepted (or at least appeared to accept) Jesus as his lord and savior.
Anyone can be baptized. Some churches practice infant baptism, and therefore the child does not have faith but is baptized on the pledge that he or she will be raised in the Christian religion. Even then, the child will eventually have to place his or her faith in Christ to receive salvation. This may or may not happen.
Some churches practice believers baptism in which one must be able to express their faith in Christ before being baptized. Even in this scenario a person could lie about having faith and is simply being baptized for some other reason (familial or societal expectation perhaps).
So as others have said, Hitler being baptized (whether as an infant or even as the head of the Nazi Party) does not disprove faith alone. It only demonstrates that the confession of the mouth must express what is truly believed in the heart.
So according to “once saved always saved”, hitler must have gone to heaven regardless of what he did during WW2.
Christians who believe in once saved always saved typically reject a belief in baptismal regeneration. Therefore, they would tell you that neither baptism nor communion have anything to do with making someone a Christian.
Furthermore, the argument they would give you is that if Hitler truly had faith then he would not have done what he did–because people who have the Love of God in their hearts don’t typically go around committing genocide.
Is this a good argument to use against people who claim “once saved always saved”?
No. People who teach once saved always saved are not Antinomians. They still believe that Christians are accountable for following moral law. They just reject the notion that someone who is truly saved can ever be lost. Christians may go through periods of trial and temptation and even fall into sin but eventually they will repent and their lives will show the fruits of sanctification.
If no sanctification is evident, they would say, its a pretty good indication that perhaps that person was not truly saved at all.