1.4. Don’t all Christian denominations make up the Church, and isn’t it just a matter of personal taste? Doesn’t God see our heart anyway?
No. The Catholic Church infallibly teaches that she alone is the true Church of Christ. Denominations that separated from her are not branches of the true Church but are separated sects. The idea that all “Christian” groups form one “church” is the condemned heresy of indifferentism and branch theory.
Pope Pius XI, in Mortalium Animos (1928), solemnly condemned ecumenism and the idea that "all religions are more or less good and praiseworthy." He affirmed that union can only occur by conversion to the one true Church.
God sees the heart — but also commands external profession of the true faith:
“With the heart, we believe unto justice: but with the mouth, confession is made unto salvation.”
Further reading:
Mortalium Animos (On Religious Unity) by Pope Pius XI - 1928