I love this question. I have wondered this myself and found answers. Here you go, from personal experience and study.
The short of it? Prophets, and prayer.
Yes, you are right, the Bible is old, but as Amos 3:7 says, "Surely the Lord God will nothing but he revealeth his secret unto his servants the prophets." God calls prophets so that people can know what He is speaking at the time. That is mainly how we have the Bible, a collection of words of God's prophets, chosen and set apart to lead the people in specific, contemporary issues at the time, and also eternal truths, mainly that of the Atonement of Jesus Christ.
Often, people will being to disobey (apostasy) and not follow God's prophets, so He withdraws them from the earth. When the people are ready again, God calls another prophet to speak His word and clarify truth. This happens all throughout the Old Testament, and a period of apostasy begins again near the end of the New Testament.
So, God just leaves us alone forever? For hundreds of years there were no prophets on the earth, no new revelation. Hence a period of reformation/enlightenment when people sought greater spiritual truths for their day, and to restore teachings of the original Christ-organized Church.
As several denominations (and non-denominational sects too) believe (albeit differently depending on where you go/who you ask), God has prophets in the Church today. But we must be careful not to blindly follow a so-called prophet, ancient or modern. How can you know?
God's values, or true eternal principles, will never change. God's truth then is God's truth now.
To know if something is right, or, as we might say, to know God's "opinion" on the subject, we can pray and ask. No promise is more repeated in canonized scripture than "Ask, and ye shall receive." Knock, and it will be opened unto you.
From my own experience I know that God answers prayers and will give you an answer to sincere prayer through the Holy Ghost. One must only have a sincere heart and faith in Christ, with real intent to act on the answer. If you have a question about any moral, doctrinal, or other issue, I know God is there, and that He is interested in you and it. He wants to answer you and He will.