This is a question that crops up every now and then. How is God knowable and how do we know that God is like this and not like that.
Historically the Christian faith has given three main ways of answering that question. Through what has been made, through what has been said and through the One who was sent.
Through what has been made
When I look at your heavens, the work of your fingers, the moon and the stars, which you have set in place, what is man that you are mindful of him, and the son of man that you care for him? (Psa 8:3-4)
For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. (Rom 1:20)
By looking at what has been made, Paul argues we can deduce things about the one who made them. Eternal power and divinity in this case. Whoever made all this is not like us (divine) and is not weak (but has power) and must be eternal (because the universe is really big and really old).
Through what has been said
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work. (2Ti 3:16-17)
We know about the character of God in the same way we know things about people – what they say and what they do. Just as people use Facebook or LinkedIn to reveal ourselves to others (not always in the same way but to showcase different aspects of our lives), the Bible is God’s self-revelation of Himself to people. We see in the pages a record of what people believe God has said and what people believe God has done.
They refused to obey and were not mindful of the wonders that you performed among them, but they stiffened their neck and appointed a leader to return to their slavery in Egypt. But you are a God ready to forgive, gracious and merciful, slow to anger and abounding in steadfast love, and did not forsake them. (Neh 9:17)
Nehemiah here calls God gracious, merciful, ready to forgive, slow to anger, abounding in love & faithful because of the way God handled the people of Israel in the Sinai wilderness. God revealed His character by the things He did. Just the way we do.
Through the One who was sent
The last and most important way He has revealed himself is through Jesus, the Son of God.
Long ago in many ways and at many times God’s prophets spoke his message to our ancestors. But now at last, God sent his Son to bring his message to us. God created the universe by his Son, and everything will someday belong to the Son. God’s Son has all the brightness of God’s own glory and is like him in every way. By his own mighty word, he holds the universe together. After the Son had washed away our sins, he sat down at the right side of the glorious God in heaven. He had become much greater than the angels, and the name he was given is far greater than any of theirs. (Heb 1:1-4)
Jesus replied: Philip, I have been with you for a long time. Don’t you know who I am? If you have seen me, you have seen the Father. How can you ask me to show you the Father? (Joh 14:9)
So very simply, if you want to know what God is like, read about Jesus.