If we are to have any hope of having truth, then something in and of itself must declare it, wholly; and if salvation is indeed to be had, and if we are to know how to love and worship God properly, then those must be revealed as well. Thankfully, according to His graces, God has provided us with His truth in His Word, the Bible, which in and of itself contains the whole truth necessary as God would have for us. It then, and it alone, must be our measuring stick, otherwise truth becomes relative, which then cannot be trusted as truth.
By its very nature, truth is absolute: a truth can only be truly one thing at one time. If a thing is entirely the color green, it cannot also be the color blue. There is no such thing as "my truth" or "your truth"; there is only THE truth.
Also by nature, the truth divides, showing what is true and what is not true. Not having the truth is having a lie. Jesus Christ, being the truth Himself (John 14:6), was a divider: Matt 10:34-35 - "Think not that I come to send peace on earth: I came not to send peace, but a sword. For I am come to set a man at variance against his father, and the daughter against her mother, and the daughter-in-law against her mother-in-law."
The Bible says that the Lord Jesus is the Word (John 1:1) and, as noted, the truth. It follows then that, if we do not have the truth as described in the Word of God (ie. we have a lie), then we do not truly have Christ. Therefore, it behooves us to make absolutely sure that we have the truth as it is brought forth in Scripture.
Paul says in 2 Cor 13:5, "Examine yourselves, whether ye be in the faith..." Being in the faith does not just mean having faith versus not having faith. It also refers to which faith we have: whether our faith is an alive faith versus a dead faith (Jam 2:17,20,26), whether we believe in another Jesus versus the true Jesus Christ (2 Cor 11:4), or whether the Gospel to which we hold is the true Gospel or not (Gal 1:6). And so to discover whether we are in the faith or not, against what can we examine ourselves? It must be against something that shows the truth of our beliefs, so that we can know whether our faith is a true faith, our Jesus is truly Jesus Christ, and our Gospel is the true Gospel. Since God's Word contains those truths, we must examine ourselves against it.
Is the way to heaven via a false faith, false Jesus or false Gospel (ie. via a lie)? How can it possibly be? And so examining our beliefs is of great importance. We had better have THE truth, or it could be to our eternal detriment.
Further, when we examine ourselves, can we say that we have a love of the truth? The Bible declares a love of the truth is given by God: 2 Thess 2:10 says, "And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved." We therefore need to pray for a love of the truth, for our benefit and because of the warning that follows in 2 Thess 2:11-12: "And for this cause God shall send them strong delusion, that they should believe a lie: That they all might be damned who believed not the truth, but had pleasure in unrighteousness."
May God's graces and mercies in this regard be with us, may God grant us a love of His Truth and Word, and may He grant us the desire and ability to honestly examine ourselves in the light of His Word.