Final answer:
The most important and longest lasting heritage of Roman Spain is Roman Catholicism.
Step-by-step explanation:
The most important and longest lasting heritage of Roman Spain is Roman Catholicism. When the Roman Empire expanded into Spain, they brought their religious beliefs with them. Catholic missionaries played a crucial role in introducing Christianity to the native populations in Spain and the New World. This legacy of Roman Catholicism continues to be a dominant religious affiliation in Spain today.