Interesting to observe the first person to enter paradise after Christ’s death was this man, a criminal. This must say something that God wanted front-and-center in how we see the results of his death.
Going for the short answer I would say the following was not essential for salvation: being baptized, observing the Lord’s supper, going to church, witnessing, reading the Bible, having a payer life, loving ones neighbor, loving God, etc. In fact not even moving his body in any particular direction was required, let alone giving to the poor or lending a hand to a person in need. He did not even have to lift a finger, literally.
However the one thing he did do was recognize that he deserved to die and called out to Jesus in faith. One single prayer for help -- guaranteed his eternity.
This is especially powerful when we imagine this man’s life was probably just one long compilation of sins. Even during the wicked times of the Roman’s, where people watched gladiators for entertainment, he knew ‘he deserved it. Yet it did not matter.
On the other hand, in a sense we see some things seemed to change for him as soon as he
confessed his faith in Jesus: He found comfort in God’s forgiveness, he happened to be at church (for Jesus was right beside him), he did not practice a life of sin after confessing his faith (even though he never got the chance), he was a witness to Christ, and he had a hope of heaven (assuming he kept believing during the next few minutes), he most likely felt love for the man Jesus, for Jesus was a kind of rescue team at the man’s darkest hour. What brave soldier is this! Comforting another man, while his own life was being sucked out of him, even while he bears the sin of the world?!
I guess that means that although nothing is required to be saved, apart form faith, anyone who does believe: wants to go to church, talk to Jesus, confess their faith, receive His comfort and have hope in heaven, etc. They will love the God that saved them.
This scene is a kind of pictorial version of this verse:
This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins. (I John 4:10)