I agree in part. The church did have the testimony of the Apostles and Prophets as witnesses to the blood of Christ during much of the First Century. Later they had the witness of the martyrs. Today, the church has the Written Word.
Your excuse that millions didn't have the Word in the first 400 years is not going to satisfy the Lord on judgment Day. He I going to ask, "Why did you call Me a liar?" And in shock you will respond, "Lord, I never called you a liar."
Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son. The Holy Bible: New International Version. (1984). (1 Jn 5:10). Grand Rapids, MI: Zondervan.
And where did God say only those who could actually READ His Word would be saved?
To BELEIVE is what matters. Why we believe does not matter to God. How we learned of Him matters not one bit.
Otherwise, no illiterate person could ever go to Heaven.
Otherwise, those who only HEARD his Word and could not read it, would never go to Heaven.