sure he was.....while the term Roman Catholic was not yet in use, if he was baptized, he was a Roman Catholic. If you are baptized, you were baptized into the Catholic church...no choice. Baptism makes you a member of the church that Jesus Christ, Himself, founded.....and that was the Catholic church. You can't be baptized a Lutheran, Methodist, Baptist.....nope, you became a Catholic. Whether or not you practice the religion of Christ is another matter. The revoutionists (reformers) decided that they could do a better job of instituting a religion than could Christ and their followers are truly fallen away Catholics....come on back, we need you and you really need us!!!
And I'm a faithful Catholic Christian....just NOT under the man-made authority of a Bishop of Rome.
The ideas and authority of the Apostles is OBJECTIVELY BEST known through their direct writing--unfiltered by 1500+ years of man-made-traditions--in what we call the New Testament of the Bible.
To be faithful to Apostolic authority and the Lord Jesus Christ, is to be faithful to the Bible, period.