There were three key developments in the process of centralizing Spain.
First was the joining of the two major Spanish kingdoms, in 1469, Aragon and Castile. These kingdoms were joined together by the marriage of Ferdinand, the heir of Aragon, and Isabella, the heiress of Castile. The couple was able to accomplish so much during their reign.
The second of the three key centralization developments was the conquest of Granada which was surrendered over to Spain in 1492. Granada was the final Moorish, Muslim, kingdom existing in Spain. The people living there were left to govern themselves with their own laws and rulers and were also able to keep their religion, but were welcome to convert to Catholicism if desired.
The Spanish Inquisition, the third key development, was an inquiry into the Muslims’ and Jews’ religious beliefs. This was held in order to see if the converted Muslims and Jews were really sincere with their new Catholic faith, because with a unity of faith the kingdom would be much easier to rule. In later years all Jews and Muslims who were not converted were expelled from Spain- a total violation of the treaty terms made in Granada.
With Spain finally centralized Isabella and Ferdinand were able to join in the discovery of the new world around them.