What do you have to do to be saved?Let's make something clear: the act of salvation is the work of God. Paul states in Romans 3 that there are no righteous, no not one, and all have fallen short of the glory of God (v. 10 and 23); he likewise states in Ephesians 2 that we are dead men and (literally in the Greek) "objects of wrath" before God (v. 1-3). What happens then is a regeneration of our heart - as Paul said: "even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved" (Eph 2:5). While we were dead, God made us alive - God had to raise us spiritually in order for us to be saved, hence sola gratia. This regeneration causes a person to confess faith in God, as the apostle John wrote:
But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God. [Jn 1:12-13]A lot of people like to quote v. 12 and forget what happens in v. 13: the apostle clarifies that a person is not a child of God because they were born by blood (that is, you're saved because you're a Jew, or because you're in a Christian family), nor by the will of flesh (that is, your works), nor by the will of man (that is, your own individual will to believe), but you are born of God - that is, by God's will. This is what our Lord meant when he told Nicodemus one had to be "born again" to see the kingdom of God (Jn 3:3). We owe nothing to ourselves and all to God the Father and the atoning work of Christ - soli deo gloria and solus christus.
Now one thing we have seen here likewise is the importance of faith. When we turn to Christ, we are justified by his blood through our faith, and we are counted as righteous in Christ. As the apostle Paul wrote: "Now to the one who works, his wages are not counted as a gift but as his due. And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness" (Ro 4:4-5).
Once you are saved, does that mean that you're automatically going to heaven?Those who are God's sheep will never be lost, as Christ said:
"I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand." [Jn 10:28-29]The apostle Paul likewise wrote:
What then shall we say to these things? If God is for us, who can be against us? He who did not spare his own Son but gave him up for us all, how will he not also with him graciously give us all things? Who shall bring any charge against God's elect? It is God who justifies. Who is to condemn? Christ Jesus is the one who died—more than that, who was raised—who is at the right hand of God, who indeed is interceding for us. Who shall separate us from the love of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or danger, or sword? As it is written, “For your sake we are being killed all the day long; we are regarded as sheep to be slaughtered.” No, in all these things we are more than conquerors through him who loved us. For I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord. [Ro 8:31-39]Now if by "automatically going to heaven" you mean that's it, you got your ticket punched and you're going to heaven no matter what you do, even if it's murder, then that's wrong. People often confuse the doctrine of OSAS with Perseverance of the Saints, but they're not the same. Perseverance does not mean you're clear to go no matter what: part of the perseverance is that you will be sanctified more and more by God, approaching closer and closer that state of glory - you'll never be sinless, but more and more you'll find you can sin less. A lot of people like to quote Philippians 2:12, but can't seem to grasp that it's only half a sentence. The full passage from Paul reads:
Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure. [Php 2:12-13]Yes, after being saved, we do things for our betterment or to show we are saved (as a pastor of mine once said, we're not "chosen to be frozen"), but it's not because of something we have to do or something we are capable of doing - it's because God is working within us and perfecting us, and He will see this through. As the apostle Paul said: "he who began a good work in you will bring it to completion at the day of Jesus Christ" (Php 1:6).
What if you commit a mortal sin after you are saved?A Christian is judged by their fruits - a person who has a heart regenerated towards God will not go out and seek to kill someone, or steal a car, or the like. He might have those temptations, he might come close, but as seen before, God will preserve him from all acts that would violate this.
I know it's not popular for some to suggest that a statement of faith can be false, or that we can judge whether or not a person is truly saved, but I believe this to be scriptural. The Lord speaks of those who call him "Lord, Lord," and yet were never known by him (Mt 7:22-23). If you study the language the apostle John uses against Diotrephes (3 Jn 1:9-11) and Jude uses against the heretics and false teachers (Jude 1:17-21), you'll see they are questioning whether or not the individuals were truly Christians to begin with.
I am getting confused. Does the person have a choice in the matter? Do they only choose once?If by choice you mean the heresy of Semi-Pelagianism where God gives a general offer and a person, by their own power, chooses, then no. If you mean does man do anything, then yes, but it is only by the grace of God. As Paul said in Ephesians 2, we are dead men before regeneration - it is God who brings us to life, not ourselves. However, once we are regenerated, then we turn and come to Christ, and are kept preserved by him. As the Lord said: "No one can come to me unless the Father who sent me draws him. And I will raise him up on the last day" (John 6:44). In the original Greek, it literally reads "no one has the power to come to me," and the drawing is not a passive thing, but is an effectual calling upon the person (the word literally means "dragging"), and the end result is that, on the last day, that person is raised up.
I'm not quite certain I know what you mean by "only choose once." A person repents and puts their faith in Christ only once, yes, though they will continue to turn to Christ for repentance and strength their entire life - again, an aspect of the "perseverance." They rest in the knowledge that they have a high priest who "always lives to make intercession for them" (Heb 7:25).