What does it mean to "call on the name of the Lord", as we find in Acts 2:21? This verse makes salvation sound so simple. Misinterpreted, every atheist who's ever stubbed his toe on a chair leg and exclaimed the Lord's name aloud would, therefore, be saved. Yet, in Matthew 7:21, Jesus also tells us "not everyone who says to me 'Lord, Lord' shall enter the Kingdom of Heaven, but he who does the will of my Father in Heaven." In other words, even so-called Christians won't be going to Heaven.
For the answer to this apparent conundrum, let us go to the third of the 10 commandments. It says “You shall not take the name of the LORD your God in vain, for the LORD will not hold him guiltless who takes His name in vain.” (Exodus 20:7)
Many have assumed that this is like using His name as a curse word (like our hypothetical toe stubbing atheist). And yes, this is one correct application. But to take God's name in vain more generally applies to any misuse of His name. If, for example, you claim "God told me...", when He did not, in fact, tell you, that is using His name in vain.
The Bible, especially the New Testament, is full of people taking the Lord's name in vain. The Pharisees, for example. Every single one of them claimed to be a follower of God, but very few of them had the faith to match their piety. Jesus even warned that a time would come when a man who killed the disciples would believe he had done a service to God (John 16:2). But back to the third commandment, let us look at the wording. It says God will "not hold him guiltless" who takes the name of the Lord in vain. It's the only one of the 10 commandments that has this clarification. Yet, taking Paul as our example, it's clear that someone who takes the Lord's name in vain, even to the point of killing Christians, can still repent and be saved.
With all this in mind, there is a "hidden" clarification to Acts 2:21. It is not hidden in the sense that you have to dig for it, as even reading the surrounding verses gives us this clarity. But it is hidden in the sense that if you just thoughtlessly read the one verse, as "casual Christians" do, you won't realise... there is no such thing as a casual Christian! Faith cannot be absent when we call upon His name, salvation is for the faithful alone. In order to be saved while calling on His name, it must not be done in vain. It cannot be the mere intellectual assent that Jesus is Lord, or that He has risen. Satan himself is fully aware of these facts. It cannot be a casual "ok, cool, this doesn't affect my life in any way. I'll just call on the name of the Lord and be saved after a nice, long life of pleasure." No, if you're going to call on the name of the Lord for salvation, it must be the most sincere thing you've ever done!
Now of course, I don't mean that with wooden literalism. "Lord, I believe; help my unbelief!" (Mark 9:24). That's sufficient. "Lord, remember me when you come into your Kingdom..." (Luke 23:42). That's enough. You don't have to be this supreme theologian or genius philosopher. But a faith that won't move you is a faith that won't save you. Calling upon the Lord cannot be empty. But if it is full of the hope that Jesus is indeed the risen Lord, who takes your sin, and calls you to righteousness, that is the call that saves.