Without a rapture, if you can never lose your salvation, what happens to a believer who takes the mark of the beast? Lose salvation?
This is one of the reasons why trying to interpret the mark of the beast as some kind of literal mark is problematic. Salvation depends solely on the work and promises of God through Jesus Christ for us.
While apostasy is very real, salvation has nothing to do with some literal "mark", but entirely to do with God's grace and Him giving, creating, and working faith in us through the Gospel (Romans 10:17, Ephesians 2:8-9). And the idea that God's word could be nullified by someone unknowingly receiving some physical mark on their body should be considered entirely absurd and inconsistent with the entire biblical witness.
A common view among many Christians and, arguably, most biblical scholars and academics, is that "the beast" of the Revelation probably refers to Nero, based on the clues that John gives us, such as that the beast's name can be numerically added up to six hundred and sixty-six, that the beast suffered a mortal wound but is brought back to life (in the form of later persecuting emperors, such as Domitian who was emperor when the Revelation was written). The case for Nero helps to also address why, going back to some of the earliest years of Christianity, the number given has a common variation: six hundred and sixteen. Essentially the name Nero Caesar adds up to 616 while Neron Caesar adds up to 666, both Nero and Neron are two forms of his name, one being the Latin form and the other the Greek form; when the name is transliterated into Semitic letters, whether Hebrew or Aramaic, it becomes NRW QSR or NRWN QSR. As mentioned, NRW QSR adds up to 616, while NRWN QSR adds up to 666. The reason why this works is because Hebrew, Aramaic, Greek, and many languages assigned numerical values to the letters of their writing system, there wasn't a separate set of symbols to represent numbers as we have today. So the Greek numeral for 1 is identical with the letter of the Greek alphabet, Alpha. So, for exmample in Hebrew and Aramaic the first letter in NRW[N] QSR is Nun, with a numerical value of 50, Resh has a value of 200, Vav has a value of 6, and so on.
In this way words and names all could have a numerical value by adding up the individiual values of each letter. It could work as a simple code. Examples of people doing this in the ancient world isn't uncommon, people could write things like, "the number of my beloved is ####", the numerical value was given which obfuscated the name. So, in this way, the name of the beast is being obfuscated by presenting its numerical value, either 666 or 616 depending on the variant manuscripts we have. John appears to suggest that his readers can figure out who he is talking (Revelation 13:18), meaning that the person/name that the figurative language of "the beast" represents is someone Christians living in the late first century knew. Remember, John wrote the Revelation to Christians living in a cluster of churches in Anatolia, specifically the Roman province of Asia which corresponds with modern south-west Turkey; the Revelation is written to Christians living in the late first century, specifically those in Asia. In the same way that Paul's letter to the Romans is written to the Christians in Rome, or the letters to the Corinthians is written to the Christians in Corinth, etc.
So that leaves the question, what is "the mark" then? Well, elsewhere in the Revelation we see that God's people also have a mark, they have God's seal. Why the forehead or right hand? This goes back to the Torah, where God through Moses instructed the Jews to have God's Law as a sign and seal upon their foreheads and right hand (which became the practice of
teffilin in Judaism). C.f. Exodus 13:9, Deuteronomy 6:11, etc.
The idea of a person having a seal speaks of allegiance, covenant, of belonging-to. The Jews were, under the Old Covenant, God's people because they were sealed with the Torah, in some cases literally as in the case of circumcision. As Christians, with the New Covenant which is in Christ, we also have God's seal. In Ephesians 1 St. Paul writes that when we came to faith we were sealed with the Holy Spirit (Ephesians 1:13). The Holy Spirit dwells in us, marks us, seals us, by God's grace and promise in the Gospel, through faith. So the people of God have a seal, a mark, in that they belong to God, we are God's because we belong to Christ and the Holy Spirit indwells us. So that through faith we are God's people, the children of Abraham, etc.
Likewise, then, those whose allegiance is to, who belong to, the beast also bear his name and mark; even as we who belong to Christ bear His name and mark.
The essence, then, is that to worship the beast or his image (which is quite literal in the context of ancient Rome, people were literally expected to regard the emperors as divine and to offer sacrifices or other religious oblations to the emperor and his image) is to belong to, be marked as, the property of the beast. One cannot belong to both Christ and Caesar, one cannot worship God and idols. For we have one God, and one Lord, even as we are one Body, and there is one Spirit, one faith, one baptism, etc. We belong to Christ alone, our Lord, we are His people, sealed in Him and by Him, as children of God and heirs of the promise.
-CryptoLutheran