If someone were to ask me why I’m not Roman Catholic, this would be my answer in 200 words or less:
I believe the Roman Catholic church has seriously erred in three fundamental areas: in its approach to God, the Bible, and salvation.
1) In its approach to God, Roman Catholicism approves the veneration of (i.e. bowing down before) images and relics, encourages praying to the saints, and promotes Mary to a semi-divine status. All of these constitute varying forms of idolatry, which Scripture condemns (cf. Ex. 20:4–5; Lev. 26:1; Acts 10:25–26; Rev. 22:8–9).
2) In its approach to the Bible, Roman Catholicism elevates church tradition to a place of authority equal to (and in practice higher than) Scripture. The Lord Jesus condemned first-century Judaism as apostate because it likewise elevated the traditions of men above the Word of God (Mark 7:6–8).
3) In its approach to salvation, Roman Catholicism adds various sacramental works to the gospel of grace. In a similar way, the apostle Paul condemned the Judaizers because they added self-righteous works to the gospel (cf. Acts 15:1–11; Rom. 11:6; Gal. 1:6–9).
These fundamental issues, in addition to a host of other doctrinal problems (e.g. purgatory, the papacy, priestly celibacy, indulgences, the Apocrypha, etc.) lead me to reject Roman Catholicism.