Laugh away, my friend. Laugh away. But it is a serious question, which those who reject the Apostolic Church face mighty difficulties in answering. The answer of traditional Christians - both Catholics and Orthodox - is very simple. Christ's Church infallibly decided this.
"What Church? Scripture reveals this Church to be the one Jesus Christ built upon the rock of Saint Peter (Matt. 16:18). By giving Peter the keys of authority (Matt. 16:19), Jesus appointed Peter as the chief steward over His earthly kingdom (cf. Isaiah. 22:19-22). Jesus also charged Peter to be the source of strength for the rest of the apostles (Luke 22:32) and the earthly shepherd of Jesus’ flock (John 21:15-17). Jesus further gave Peter, and the apostles and elders in union with him, the power to bind and loose in heaven what they bound and loosed on earth. (Matt. 16:19; 18:18). This teaching authority did not die with Peter and the apostles, but was transferred to future bishops through the laying on of hands (e.g., Acts 1:20; 6:6; 13:3; 8:18; 9:17; 1 Tim. 4:14; 5:22; 2 Tim. 1:6).
By virtue of this divinely-appointed authority, the Catholic Church determined the canon of Scripture (what books belong in the Bible) at the end of the fourth century. We therefore believe in the Scriptures on the authority of the Catholic Church. After all, nothing in Scripture tells us what Scriptures are inspired, what books belong in the Bible, or that Scripture is the final authority on questions concerning the Christian faith. Instead, the Bible says that the Church, not the Scriptures, is the pinnacle and foundation of the truth (1 Tim. 3:15) and the final arbiter on questions of the Christian faith (Matt. 18:17). It is through the teaching authority and Apostolic Tradition (2 Thess. 2:15; 3:6; 1 Cor. 11:2) of this Church, who is guided by the Holy Spirit (John 14:16,26; 16:13), that we know of the divine inspiration of the Scriptures, and the manifold wisdom of God. (cf. Ephesians 3:10)."
Taken from:
https://www.scripturecatholic.com/catholic-faith/