I have to quote one of my favorite apologists:
1. Best One-Sentence Summary: I am convinced that the Catholic Church conforms much more closely to all of the
biblical data, offers the only coherent view of the
history of Christianity(i.e., Christian, apostolic Tradition), and possesses the most profound and sublime Christian morality, spirituality, social ethic, and philosophy.
2. Alternate: I am a Catholic because I sincerely believe, by virtue of much cumulative evidence, that Catholicism is
true, and that the Catholic Church is the visible Church divinely-established by our Lord Jesus, against which the gates of hell cannot and will not prevail (
Mt 16:18), thereby possessing an
authority to which I feel bound in Christian duty to submit.
3. 2nd Alternate: I left Protestantism because it was seriously deficient in its interpretation of the Bible (e.g., "faith alone" and many other "Catholic" doctrines - see evidences below), inconsistently selective in its espousal of various Catholic Traditions (e.g., the Canon of the Bible), inadequate in its ecclesiology, lacking a sensible view of Christian history (e.g., "Scripture alone"), compromised morally (e.g., contraception, divorce), and unbiblically schismatic, anarchical, and relativistic. I don't therefore believe that Protestantism is all bad (not by a long shot), but these are some of the major deficiencies I eventually saw as fatal to the "theory" of Protestantism, over against Catholicism. All Catholics must regard baptized, Nicene, Chalcedonian Protestants as Christians.
4. Catholicism isn't formally
divided and sectarian (
Jn 17:20-23;
Rom 16:17;
1 Cor 1:10-13).
5. Catholic
unity makes Christianity and Jesus more believable to the world (
Jn 17:23).
6. Catholicism, because of its unified, complete, fully supernatural Christian vision, mitigates against
secularization and humanism.
7. Catholicism avoids an unbiblical
individualism which undermines Christian community (e.g.,
1 Cor 12:25-26).
8. Catholicism avoids
theological relativism, by means of dogmatic certainty and the centrality of the papacy.
9. Catholicism avoids
ecclesiological anarchism - one cannot merely jump to another denomination when some disciplinary measure or censure is called for.
10. Catholicism formally (although, sadly, not always in practice) prevents the theological
relativism which leads to the uncertainties within the Protestant system among laypeople.
http://www.ourcatholicfaith.org/reasons.html