In a nutshell

To keep pride out of heaven God made salvation a free gift of His grace to all who believe in Jesus. Jesus said "Simply believe", because this separates the wheat from the chaff, the proud heart from the humble. If salvation was a list of rules to complete then the proud could simply follow them and get in. Instead Jesus fulfilled the law of requirements so the humble would believe in Him and the proud would believe that God is rewarding them for how great they are.