Pilate asked that question, and I imagine everyone has asked the very same. But instead of asking What, it would serve us better to ask Who. Who is Truth? And if I say that God is Truth, I must then say why I believe? Some may disagree, but I find it difficult to claim belief in anything without an answer to, 'Why believe?'
If I'm going to believe in God, I have to know why I choosing to believe. If I'm going to accept His word on anything, I need to know I can trust His word. This necessitates looking in His word, the Bible.
There are many truths in the Bible-- and, perhaps, more important -- than the one I'm going to discuss here, but in order to know what to believe, I first have to know I can trust God, and there are three verses that articulate God's trustworthiness.
As to the first:
Num 23:19 God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good?
Here God's word says He is not like sinful men, who lie, or bear false witness. This is good to know. God is basically saying, "Eric, I'm not like you, I won't lie to you." God forgive me for putting words in His mouth.
As to the second:
Titus 1:2 In hope of eternal life, which God, that cannot lie, promised before the world began;
First we have, God should not lie, here now it's strengthened: God cannot lie. And now, as to the third:
Heb 6:18 That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:
And this is where God's faithfulness is sealed. It is impossible for God to lie. God. The omnipotent God, the all powerful God. For all his power there is actually something He cannot do. It is in fact impossible for Him.
It is impossible for God to lie, therefore he cannot lie. But even more, he has no desire to lie. Has no need to lie. This is not a shortcoming, or character flaw, however. It is simply not in His Nature to lie. He is Holy. Pure. The epitome of Cleanliness. God cannot lie anymore than a goldfish can leap out of its bowl and strum a Spanish guitar and sing Mozart's The Marriage of Figaro. It is not in the goldfish's nature, and therefore cannot do it. The same is true of God. And God is not made less because of it.
So why is this important? Knowing God cannot lie, that it is impossible for him to lie, provides us any number of assurances. For example: Many denominations will tell you that through sin you can lose the salvation God gives you through the blood of His Son Jesus.
Rom 10:9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. [10] For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. [11] For the scripture saith, Whosoever believeth on him shall not be ashamed.
With the heart man believes unto righteousness, and with the mouth confession is made unto salvation.
John 10:28 And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
Since God cannot lie, and Jesus is God made flesh, Jesus himself cannot lie, and Jesus said "they shall never perish..."
Eph 2:8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: [9] Not of works, lest any man should boast.
And since there is nothing you can do to earn eternal life, and Jesus says you will never perish, nor can any man pluck you out of His hand, your place in eternity is assured.
Some will say you can walk away from God, reject your salvation. I reject that belief. God cannot lie, therefore if the above verses are true, any other verse must mean something else, another context. It's as simple as that. The prodigal son didn't stop being his fathers son simply because he took his share and squandered it. Notice too, the brothers reaction: Anger, Resentment, Jealousy, but the Father puts it all to rest with this:
Luke 15:31 ...Son, thou art ever with me, and all that I have is thine. [32] It was meet that we should make merry, and be glad: for this thy brother was dead, and is alive again; and was lost, and is found.
Do Christians sin? Of course they do. Do they stray from the straight and narrow? Of course. Do we lose the love and gifts of our Father? Namely, Eternal Life? No.
Apply everything you read in the Bible with this standard: "God will never lie to me." Do this, comparing one verse to the other, and you can know the truth of what God is saying to you.
God Bless