You'll forgive me if I center my answer on the Bible: A Christian really has no other recourse. Logic demands that if God exists then He has definitive attributes. Common sense demands that if a truly transcendent, infinite, and eternal being exists there is no way that we (temporal, finite, limited, etc.) could know anything about it's existence or being unless it has revealed itself and we perceive and accept the revelation.
for whoever would draw near to God must believe that he exists and that he rewards those who seek him. - Hebrews 11:6b
This is a fundamental reason why Biblical revelation is necessary for the knowledge of God but it is not the final authority. The Bible's main focus is the revelation of Jesus Christ because it is He who reveals God the Father in whatever detail we mortals are able to grasp.
And the Father who sent me has himself borne witness about me. His voice you have never heard, his form you have never seen, and you do not have his word abiding in you, for you do not believe the one whom he has sent. You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me, yet you refuse to come to me that you may have life.- John 5:37-40
There are a great many people who consider themselves to be Christians. Some claim Christianity based upon membership in and practice of a particular denomination or religious system. Some claim Christianity based upon upbringing, having been raised in a Christian household or a Christian nation (if you can conceive of that). But there are those who have and who do actually experience God as Father by the reception of Jesus Christ as Lord and Savior.
because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. For the Scripture says, “Everyone who believes in him will not be put to shame.” - Romans 10:9-11
This is the critical difference between human religion and Christianity: Christianity is an actual personal relationship with God. There is a new birth (birth from above) that God brings about when a sinner renounces hope in all other means and cries out in his heart for the blood of Christ to be applied to his account before God. That person is then justified before God by grace through faith in Christ's works rather than their own works and the Spirit of that very same God takes up residence within that person and testifies to the reality of both God and their salvation.
For we ourselves were once foolish, disobedient, led astray, slaves to various passions and pleasures, passing our days in malice and envy, hated by others and hating one another. But when the goodness and loving kindness of God our Savior appeared, he saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit,
whom he poured out on us richly through Jesus Christ our Savior, so that being justified by his grace we might become heirs according to the hope of eternal life. - Titus 3:3-7
It is because of the Spirit of God living within a born again believer (Christian), opening their minds to understand the things of God, and striving within them against their sinful nature so that they can obey in their hearts that a Christian can say that they KNOW God. This is not knowledge in an intellectual sense but in an intimate, relational, and personal sense.
Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son. And this is the testimony, that God gave us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life. I write these things to you who believe in the name of the Son of God, that you may know that you have eternal life. - 1 John 5:10-13
As Codosaur has indicated in (his?) answer Biblical literacy within Christendom as a whole is appallingly low so I suppose that in general much surprise will be experienced by everyone when we come face to face with God. In fact, the Lord Jesus will say to many who think that they belong to Him that He never knew them:
“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’ - Matthew 7:21-23
And, as the Bible reveals everything that we need to know about God in order to understand our need to be saved by Him and the means He has provided to that end but does not claim to reveal everything that there is to know about the totality of God, it is certain that everyone, even the most faithful born again believer, will learn a great many things when they come fully into His presence.
When I was a child, I spoke like a child, I thought like a child, I reasoned like a child. When I became a man, I gave up childish ways. For now we see in a mirror dimly, but then face to face. Now I know in part; then I shall know fully, even as I have been fully known. - 1 Corinthians 13:11-12
What will not happen is the discovery that God is the opposite of what He has revealed Himself to be in the Bible and in Christ Jesus. We will not discover that God is finite, or petty, or ignorant, or less than good. We will not discover that He is anything like us.
Christians are certain that the Bible is true because the God that inspired the writing of it lives within them. A Christian has the apparently audacious ability to say that they know God AND be correct.
He who comes from above is above all. He who is of the earth belongs to the earth and speaks in an earthly way. He who comes from heaven is above all. He bears witness to what he has seen and heard, yet no one receives his testimony. Whoever receives his testimony sets his seal to this, that God is true. For he whom God has sent utters the words of God, for he gives the Spirit without measure. The Father loves the Son and has given all things into his hand. Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him. - John 3:31-36