Wherefore putting away lying, speak every man truth with his neighbour: for we are members one of another.
- Ephesians 4:25
The truth is God's word, according to Jesus Christ.Every word that is not God's word, or that contradicts God's word, is a lie, and he who speaks lies is a liar.
A man who fills up his speeches with speculations about God's word is a liar, and if he does so intentionally, he is a deceiver. He takes a place along side God's arch-enemy, the Devil, in speaking lies in hypocrisy, in order to deceive and trick people. They do this to get something out of the people they deceive, without offering anything worthwhile in return. Hence they are liars and thieves.
You might as well say that they (the Devil and his spokesmen) are in league together, conspiring to hinder the purposes of God, among which is his desire that every man and woman should come unto the knowledge of the truth.
...speak every man truth with his neighbor: for we are members one of another.
Your neighbor certainly includes those who are partakers with you of the grace of God, who are born from above, are part of the family of God, having the same lord Jesus Christ and the same God and Father. Your neighbor has the same spirit from God as you do, and the same eternal life. All those not part of the family of God can benefit from the truth of God's word too, since the hearing and believing of God's word is required to become a part of the family.
Truth is always most profitably spoken with respect and love for the one, true God. If done in this manner, you see the added benefit mentioned in Ephesians 4:15:
...speaking the truth in love, may grow up into him in all things, which is the head, even Christ...