Well, prior to the establishment and completion of God's word, the Bible, there were a variety of means whereby God declared what He was going to do and what He wanted the recipients of His communications to do. God used visits from angels, visions, dreams, prophets, a burning bush, stone tablets, a disembodied hand writing on a wall, a talking donkey, even, to express His will to His creatures. God spoke to Elijah in a still, small voice from the mouth of the cave in which the prophet was hiding, and he led the people of Israel with a pillar of flame at night and pillar of cloud by day. God knocked Paul to the ground and blinded him and delivered Jonah to Nineveh in the belly of a great fish. And so on. When God wanted to communicate Himself to someone, He never failed to do so and they never failed to understand who it was who was communicating with them.
Today, however, God has given to us the revelation of Himself in His word, the Bible. It is complete, providing to us all we need to "live godly in Christ Jesus."
2 Timothy 3:16-17
16 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness;
17 so that the man of God may be complete, fully-equipped for every good work.
Psalm 119:105
105 Your word is a lamp to my feet And a light to my path.
1 Peter 2:2
2 like newborn babies, long for the pure milk of the word, so that by it you may grow in respect to salvation,
In tandem with the Bible, and more vital, is the Person of the Holy Spirit, who teaches God's children the "deep things of God." Without his convicting, illuminating, enabling work in the hearts and minds of born-again believers, they would be like sheep without a shepherd, wandering into all sorts of foolishness, sin and destruction.
John 14:26
26 "But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you.
John 16:13
13 "But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come.
1 Corinthians 2:10-16
10 For to us God revealed them through the Spirit; for the Spirit searches all things, even the depths of God.
11 For who among men knows the thoughts of a man except the spirit of the man which is in him? Even so the thoughts of God no one knows except the Spirit of God.
12 Now we have received, not the spirit of the world, but the Spirit who is from God, so that we may know the things freely given to us by God,
13 which things we also speak, not in words taught by human wisdom, but in those taught by the Spirit, combining spiritual thoughts with spiritual words.
14 But a natural man does not accept the things of the Spirit of God, for they are foolishness to him; and he cannot understand them, because they are spiritually appraised.
15 But he who is spiritual appraises all things, yet he himself is appraised by no one.
16 For who has known the mind of the Lord, that he will instruct him But we have the mind of Christ.
Together, the Holy Spirit and the Scriptures provide to us all we need to know and understand of God's will and how to walk in His way.