What was the importance of religion in the conflict between the English and the Spanish?

If you are referring to the period of the Spanish Armada, the issue was the restoration of the Catholic faith to England. King Henry VIII had established a 'national' church with the King of England at its head instead of the Catholic church ruled by the pope in Rome.

Philip II of Spain, who sent out the Armada, considered himself the defender of the Catholic faith in Europe. And because he had been married to the late Queen Mary of England, he saw restoration of the Catholic faith as a special responsibility where England was concerned.

On a political level, protestant England was actively supporting her fellow Protestans in The Netherlands, who were at the time in full revolt against his rule. Defeating England and its protestant Queen Elisabeth I would have taken care of that problem too.