The evangelist John asks a very challenging question in his first epistle (1 John 2:22). He asks, 'Who is a liar but he that denieth that Jesus is the Christ?
He goes on to say, 'He is an anti-christ, that denieth the Father and the Son. Whosoever denieth the Son, the same hath not the Father:'
Strong words from a man who preached a God of love!
But, how does one arrive at the position that a man is a liar if he denies that Jesus is Christ? I guess the opening premise is that God is Truth [Deut. 32:4 etc]. From here one can reasonably deduce that God's Word is true, and that Christ, coming from God, is the Word of God [Rev. 19:13]. If this be the case, then a denial of Christ is a denial of truth [John 14:6]. To deny truth is to make truth a lie. Is this, therefore, the unforgivable sin?
Christ deniers really have no rock to stand on when 'truth' becomes a victim, as happens in war. Where is the hope of justice if there is no God to hear your cry? If your daughter is raped and murdered by soldiers, or your son tortured, tied and shot in the back of the head, how do you respond? Hate and seek revenge? Add your vitriol to the great hell of war? Seek justice from courts that may never provide justice? Give up on life itself, with no hope of glory?
I'm with Paul, who said, 'For me to live is Christ, and to die is gain.' [Phil.1:21]