There are numerous examples of Christians and their countries who abandon Jesus and God completely.
The peoples of Russia and Germany were once fervant Christians before they changed, become anti-Christs and murderers of innocents on a massive scale.
The BIG QUESTION is, " Are they still saved because they believed in Jesus at one time in their lives?
The first example I'll give is that of Communist Russia. Russia was a fervently Christian/Orthodox country for centuries. Then came the Communist uprising in which they turned from God/Jesus and became an atheist state. They murdered and persecuted many Christians. Former Christians in Russia fully supported this murder and turning from Jesus!
The second example is Christian Germany. When Hitler came to power, Germany was a Christian nation. But with the Nazis, Jews were hated/persecuted/murdered by the millions. Germans who used to be Christians could no longer accept Jesus and turned from Him. Why? Because Jesus was a Jew and the Nazi Germans couldn't accept a Jew for a God. They invented an Aryanized Jesus who wasn't Jewish at all. The German people turned from Jesus enmasse!
So, the question is, are these examples of peoples who once were true Christians and turned anti-Christ and murderers still saved because they once believed in Jesus?
Did Jesus forgive all these their sins and take them to heaven even though they were unrepentant sinners?
If they turn away from God and Jesus then they are not saved and if they turn away from Jesus but still believe in a God whether the God of Israel or another god they are not saved.
Heb 6:1 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
Heb 6:2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
Heb 6:3 And this will we do, if God permit.
Heb 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Heb 6:5 And have tasted the good word of God, and the powers of the world to come,
Heb 6:6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
If a person believes in Jesus as Lord and Savior but then stops believing and believes in an alternate reality whether atheist or another religion then they cannot be saved and they cannot accept the truth and be saved again for God will not allow them to go through the process of being saved again.
When a person stops believing in Jesus then they forfeit forgiveness of their sins and their sins are then on their record and they cannot come back to the truth and believe Jesus is Lord and Savior for God will not allow them to be saved again.
If a person lives in sin and repents then their sins are forgiven and they can be saved but if they fall away which means they abandon the truth to believe in something else then they forfeit salvation which cannot be applied in their life anymore.
1Ti 4:1 Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
1Ti 4:2 Speaking lies in hypocrisy; having their conscience seared with a hot iron;
1Ti 4:3 Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth.
1Ti 4:4 For every creature of God is good, and nothing to be refused, if it be received with thanksgiving:
1Ti 4:5 For it is sanctified by the word of God and prayer.
A good example is the new age movement that shall come out in the latter times which is now which is an interpretation of the Bible based on the occult and evolution and people are still evolving to be spiritual, and Jesus is not Lord and Savior but a good teacher and love and evolved to be an ascended master, and believe in no personal God but honor the God of forces or the power of nature as their higher power the evolutionary process, and the New Age Christ is the final teacher.
Which the Bible says some shall depart from the faith the truth of the Bible and embrace the new age movement interpretation of the Bible and religions so they cannot come back to the truth and be saved.
But if a person is in the new age movement but has not embraced the truth and embrace the truth then they can be saved for they did not depart from the truth.
God said the time will come when the world will not want to listen to the truth of the Bible but want to hear it according to their own lusts which is when the New Age Christ starts working in the world which he will push the agenda of the new age movement and some people will depart from the truth and embrace the new age movement.
The new age movement is the cause of why the world rebels against God.
Some people believe in a pre-tribulation resurrection of the saints but Paul said let no person deceive you for the gathering of the saints unto Christ shall not happen until the man of sin claims to be God which is in the middle of the 7 years period God is allowing the world to have their way and rebel against Him so He can end sin on earth.
When the resurrection does not happen before the 7 years period it will overthrow the faith of some people that believe in the resurrection happening before the 7 years period and they will embrace the new age movement interpretation of the Bible and religions which they cannot come back to the truth.