This may be the most important question one can ask, and it was asked by the Philippian jailor in Acts 16:30 Paul answered, "Believe in the Lord Jesus, and you will be saved"

But despite the fact that Paul elaborated upon this, such as in Romans, there is a great deal of disagreement among the Christian community as to what it means.

What it means, first of all, is that salvation is by faith apart from works. Paul elaborates in Romans 4

1  What then shall we say that Abraham, our forefather, discovered in this matter?
2  If, in fact, Abraham was justified by works, he had something to boast about— but not before God.
3  What does the Scripture say? "Abraham believed God, and it was credited to him as righteousness."
4  Now when a man works, his wages are not credited to him as a gift, but as an obligation.
5  However, to the man who does not work but trusts God who justifies the wicked, his faith is credited as righteousness.
6  David says the same thing when he speaks of the blessedness of the man to whom God credits righteousness apart from works:
7  "Blessed are they whose transgressions are forgiven, whose sins are covered.
8  Blessed is the man whose sin the Lord will never count against him."

Upon believing the promise of God one is saved, and that apart from doing anything. Thus salvation is finalized upon a person trusting in the Lord Jesus Christ. And likewise Jesus said, "I tell you the truth, whoever hears my word and believes him who sent me has eternal life and will not be condemned; he has crossed over from death to life." John 5:24 Upon coming to trust in Christ, it's a done deal and the person's fate is eternally secure.

But notice also Paul says Lord Jesus. To believe in Jesus is not only to believe in what he said, but also to believe in the one who said it. One cannot believe the promise of God and yet reject Jesus as Lord. "If you confess with your mouth, "Jesus is Lord," and believe in your heart that God raised him from the dead, you will be saved." Rom 10:9 But what does "Lord" mean? Jesus himself said, "But why do you call Me ‘Lord, Lord,’ and do not do the things which I say?" Lk 6:46  To call Jesus "Lord" is to pledge allegiance to him to do what he says. It is embracing a subordinate role to Jesus as Lord, looking to him for instructions, commands, meaning and purpose in life. However, this allegiance itself is not a work, but rather an attitude. Salvation is not contingent upon works, but attitudes.
In my experience and research over the decades it appears that the vast majority of Christians don't believe the gospel. They tend to either reject salvation being contingent upon accepting Christ as Lord or reject the idea of salvation being by faith apart from works and that the Lordship condition means salvation by works, and they misread scripture to that end. This second group makes up the majority of the Christian community. They view salvation as conditioned upon one's ongoing performance. Among them are Catholics, Orthodox, Pentecostals, Methodists, Church of Christ and more.

Free Gracers

In contrast to the performance based salvation Christians are the Free Gracers. These embrace salvation by faith apart from works, but reject the Lordship of Christ as a condition for salvation. Most churches with the word "grace" in their name are free grace churches, such as "Grace Chapel" in Lexington, MA. But free gracers are also found among non-denominational Churches and among the Brethren churches, followers of Darby, and Witness Lee's Local Church, also known as the "Recovery Movement".

Reformed Theology

Then there are those of a Reformed Theology, namely Calvinists and Lutherans. For them a person is fated to eternal life prior to birth and as such the elect are never in their life in danger of going to hell as they are predestined prior to birth to go to heaven and nothing can change that fate. Thus logically Reform Theology leads to the idea that salvation is not by faith, nor by works, nor by one's allegiance to Christ as Lord, but rather by a pre-birth election. While the elect, according to the theory, do come to faith in Christ at some point, that doesn't change their fate, as they were already predestined to be saved. Thus Reformed theology rejects all the conditions for salvation mentioned in the presentation of the gospel.

However very few who claim to be aligned to such theology actually believe it. That is, for example, very few who claim to be Calvinists actually apply Calvinism. Those who do are called Hyper-Calvinists. Typically Calvinists and Lutherans will put aside reasoning, thinking, rationality in matters of theology, which they have to do in order to avoid considering the logical conclusions of their theology. While they will typically give the same answer as Paul gives above, they abandon their own theology to do it. Not sure what to think of these hypocrites. As they "believe" in Reformed theology in name only, it's likely they "believe" the gospel in name only.