Paul tells Titus:
Paul, a servant of God, and an apostle of Jesus Christ, according to the faith of God's elect, and the acknowledging of the truth which is after godliness; in hope of eternal life, which God, that cannot lie, promised before the world began; but hath in due times manifested his word through preaching, which is committed unto me according to the commandment of God our Saviour;(Titus 1:1-3). This passage, among many others that could be provided, demonstrates that God, and especially Jesus Christ, is made manifest through the preaching of the Word of God. We are not Paul, but we too preach the gospel and bear witness to the truth.
Paul has another explanation of the way by which this knowledge of God is conveyed in Romans 10:
For the scripture saith, Whosoever believeth on him shall not be ashamed. For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon him. For whosoever shall call upon the name of the Lord shall be saved. How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher? And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things! But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report? So then faith cometh by hearing, and hearing by the word of God. But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world. But I say, Did not Israel know? First Moses saith, I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you. But Esaias is very bold, and saith, I was found of them that sought me not; I was made manifest unto them that asked not after me. But to Israel he saith, All day long I have stretched forth my hands unto a disobedient and gainsaying people.(Romans 10:11-21).
One of the points that Paul is making is that the knowledge of God is by faith, faith in the preached word of God. There is sadly today a school of apologetics that seems bent on undermining or downplaying the necessity of faith for the knowledge of God. I don't think the motivation of those men is to undermine the necessity of faith, instead I think they have the commendable motivation of trying to provide a more persuasive defense (apologetic) of the faith.
"You're asking them to just believe that the Bible is God's word?" they ask incredulously, but that is mostly right. I'm not asking folks to believe it on blind faith. There are certainly reasons to recognize the Bible as God's word. These reasons, though, don't amount to "proof." If one could prove the faith, it would not be faith.
Recall, for example, when the Corinthians wanted proof that Christ was speaking in Paul. What did he tell them?
This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established. I told you before, and foretell you, as if I were present, the second time; and being absent now I write to them which heretofore have sinned, and to all other, that, if I come again, I will not spare: Since ye seek a proof of Christ speaking in me, which to you-ward is not weak, but is mighty in you. For though he was crucified through weakness, yet he liveth by the power of God. For we also are weak in him, but we shall live with him by the power of God toward you. Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates? But I trust that ye shall know that we are not reprobates.(2 Corinthians 13:1-6).
His proof is their faith! Paul doesn't appeal to the authority of men (as the Romanists do) or to an attempted logical proof (as some of our misguided brethren seem to do) but to the testimony of the Spirit in the life of the believers. They know it is the word of God by faith, by the testimony of the Spirit and the experience of the power of the Spirit in their life.
That brings us to the second aspect of how we know God: the testimony and work of the Holy Spirit. The Prophet Jeremiah put it thus:
But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.(Jeremiah 31:33 - quoted as well in Hebrews 8:10 and 10:16).
Paul explains that some portion of this is, by general revelation to all mankind, written in the hearts of all men:
Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;)(Romans 2:15).
We see again this internal testimony of the Spirit in Paul's epistle to the Romans: "The Spirit itself beareth witness with our spirit, that we are the children of God:" (Romans 8:16).
And John tells us the same thing: "This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth." (1 John 5:6)
Scripture explains as well that that this is the heart-opening work of the Spirit:
And a certain woman named Lydia, a seller of purple, of the city of Thyatira, which worshipped God, heard us: whose heart the Lord opened, that she attended unto the things which were spoken of Paul.(Acts 16:14).
This is the work of regeneration, as Paul teaches:
For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit. For to be carnally minded is death; but to be spiritually minded is life and peace. Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. So then they that are in the flesh cannot please God. But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.(Romans 8:5-9).
And the fruit of the Spirit's work includes, among other things, faith:
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law.(Galatians 5:22-23)
As indeed, all goodness, righteousness and truth comes from him: "(For the fruit of the Spirit is in all goodness and righteousness and truth;)" (Ephesians 5:9).
So then, what is the proof of the faith? What is it that makes the knowledge of God certain? It is faith. Scripture declares: "Now faith is the substance of things hoped for, the evidence of things not seen." (Hebrews 11:1).
Through the power of the spirit, men receive (by faith) the word of God as such, and not merely as men's words: "For this cause also thank we God without ceasing, because, when ye received the word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the word of God, which effectually worketh also in you that believe." (1 Thessalonians 2:13)
And as a result of this faith, we can have full confidence and complete assurance of what is said, because the words of Scripture are the words of God:
Moreover I will endeavour that ye may be able after my decease to have these things always in remembrance. For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. And this voice which came from heaven we heard, when we were with him in the holy mount. We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts: knowing this first, that no prophecy of the scripture is of any private interpretation. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.(2 Peter 1:15-21)
It is the Spirit of God that permits us to hear, to know, and to remember the Word of God. It is by faith in him that we can know anything with absolute confidence, for it is written: "God forbid: yea, let God be true, but every man a liar; as it is written, That thou mightest be justified in thy sayings, and mightest overcome when thou art judged." (Romans 3:4) and again "For if our heart condemn us, God is greater than our heart, and knoweth all things." (1 John 3:20). God's testimony on something is more to be trusted than that of any man, even the consensus of mankind, and even than our own heart. We must strive to have the faith of Job, who declared: "Though he slay me, yet will I trust in him" (Job 13:15). For God has delivered, is delivering, and will deliver us from the second death (2 Corinthians 1:10 Who delivered us from so great a death, and doth deliver: in whom we trust that he will yet deliver us; ) of which we may have confidence through the work of Christ (Ephesians 3:12 In whom we have boldness and access with confidence by the faith of him. & Acts 17:31 Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.)
Will men claim this is "fideism" and "anti-intellectual"? I cannot say, and while derogatory words can sting, I will press on and faithfully declare the gospel delivered by the apostles, evangelists, and prophets of old.