You can't join if you're a Catholic because, among other reasons, the Catholic Church considers Freemasonry to be conspiratorial, an organisation seeking to subvert the state, and anti-Catholic too, essentially a living, breathing graven image. The "New Catholic Encyclopedia" states that "Freemasonry displays all the elements of religion, and as such it becomes a rival to the religion of the Gospel. It includes temples and altars, prayers, a moral code, worship, vestments, feast days, the promise of reward and punishment in the afterlife, a hierarchy, and initiative and burial rites" (vol. 6, page 137).
"Catholicism is essentially a revealed religion; it is essentially supernatural, both in its destiny and in its resources. Beyond all natural fulfillment, it tends toward an eternity of ineffable union with God in Himself; and beyond all natural resources, it begins that union here and now in the sacramental life of the Church. ...
Masonry, on the other hand, is essentially a religion of "reason." With an insistence and a consistency matching Catholicism's self-definition, Masonry promises perfection in the natural order as its only destiny -- as indeed the highest destiny there is. And it provides for this perfectibility with its resources: the accumulated sum of purely human values, subsumed under the logo of "reason." " (Man as God, very simply put)
The First Commandment is the basis of the Catholic Church's opposition on philosophical grounds 'I am the Lord thy God; thou shalt not have false gods before me'
The second objection relates to the oaths members must take because they violate the Second Commandment (Catholic order of commandments may differ from protestant) "You shall not take the name of the Lord your God in vain"; invoking without adequate cause the name of God.
The Catholic Church advocates neighbourly tolerance for individual Masons, but joining the craft is forbidden.
Vinvinoveritas, are you talking about the Scottish Rite?