We know the carnal mind is a hostile enemy (enmity) against the Spirit and can not perceive Spiritual things.
Romans 8:5 For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.
Romans 8:6 For to be carnally minded is death; but to be spiritually minded is life and peace.
Romans 8:7 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.
Romans 8:8 So then they that are in the flesh cannot please God.
Can a rational mind understand heavenly things, no. Even though our human mind cannot comprehend the whole mystery of God, He let Himself to be grasped by our mind. No human mind can fully understand God, but to those who are opened to enter into His mystery He will reveal himself enough to give you the certitude conviction that He is real.
John 3:11 Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.
John 3:12 If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?
Belief in God is rational only if there is sufficient evidence for the existence of God, but the carnal mind can not rationalize God for it will not allow the mystery of God to be revealed. This is why many refuse to hear His word that has gone out into the world through every generation. Is it God's fault that they did not hear, or is it their own fault as many quit seeking Him. Faith does come by hearing and hearing by the word of God, yet many refuse to hear His still small voice calling them.
Romans 1:20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
Romans 1:21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
Romans 1:22 Professing themselves to be wise, they became fools,
Romans 1:23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.
No one can stand before God in His judgement without excuse.