The law states (Recopilacion, lib. vi, tit. ii, ley xii):


"The islands of Mindanao are adjacent to those of the district of the Filipinas. Their natives, who have adopted the religion of Mahomet, have rebelled; and since then, in alliance with the enemies of this crown, they have done great harm to our vassals. In order to facilitate their punishment, it has been deemed an efficacious corrective to declare that those who should be captured in war be made slaves. 


"We order that such be done; but that this distinction be observed, namely, that if the said Mindanaos be simply heathen, they be not regarded as slaves; but if they be Moors by nation and birth, and shall go to other islands to introduce their dogmas or teach their Mahometan religion, or make war on the Spaniards or Indians who are subject to us, or hinder our royal service, then in that case they may be made slaves. But those who are Indians and shall have become Mahometans shall not be made slaves. Such will be persuaded by lawful and kind methods to be converted to our holy Catholic faith."


A general law that prohibited slavery of Filipino natives, which was extended expressly to the Moluccas by a special law, was promulgated by Felipe III, in Madrid, on October 10, 1618.



Sources: 

<ol class="article-sources">
<li>
The Philippine Islands, 1493-1803; explorations by early navigators, descriptions of the islands and their peoples, their history and records of the Catholic missions, as related in contemporaneous books and manuscripts, showing the political, economic, commericial and religious conditions of those islands from their earliest relations with European nations to the beginning of the nineteenth century; (Volume 35), Emma Helen Blair, et., al.
</li>
</ol>

July 4, 1570: Filipe II legalized the taking of slaves in Mindanao by the Spaniards