(Titus 1:1-2 ESV)
[1:1] Paul, a servant of God and an apostle of Jesus Christ, for the sake of the faith of God's elect and their knowledge of the truth, which accords with godliness,  in hope of eternal life, which God, who never lies, promised before the ages began...
Two things to catch here:
1) God never lies
2) God (who never lies) promised
Let's just think about this for a few moments. God never lies! It is against his nature to do so. And this God, who never lies, also promised! Oh how much surer and truer eternal life is since this is so.
The very character of God is one of absolute truth. His holiness and perfection has no room for any lie. The smallest of lies is worth an eternity in hell facing the wrath of God. He never lies - in fact Hebrews 6:18 tells us that 'it is impossible for God to lie.' This is His character - part of who He is as God. And if he never lies, then we can take him at his word without ever questioning him.
Some of us are blessed to have friends who rarely lie (because we are all liars). They're proven themselves worthy to be believed; their yes' are yes and their no's are no (for the most part). When they say they will be there at a certain time or say they will get something done, you know they will. But we have a God who never lies. When He speaks, only truth comes forth.
And our God does not stop there. But to reassure us, our God, who never lies, PROMISED. When the trustworthy friend who rarely lies also promises something, you know for certain that he or she will come through. But when the perfect God, who never lies, promises something... then there is no denying whatsoever that the things He says are true! (Because there was no denying it even in the first place).
This is comforting and incredible is it not? Every promise God makes in the Bible is true. It is His word. He never lies and yet He gives us an oath/promise to reassure us and make our feeble human hearts even more certain of his word.
What a great God and what great promises we have received and look forward to receiving in full.