We believe because God has shown Himself to us through faith, graced in many ways, many people, and many signs. We also have unbelief, however, because we are weak, finite, little creatures--but most of all because, we walk the wrong paths, listen to the wrong people, and seek solace, and Truth, in the wrong signs.
Yet, the good news of the Gospel is that the Good Shepherd, the Messiah, Jesus Christ, will seek us out, even in the smallest mustard seed of our belief, within the deepest recesses of our soul, meeting us forgivingly, and offering us Life in Him, even if it takes His dying for our Life.
With the Cross as our backdrop, and horizon, we do believe. But why do we also harbor unbelief?
"Who among you is wise and understanding? Let him show his works by a good life in the humility that comes from wisdom. But if you have bitter jealousy and selfish ambition in your hearts, do not boast and be false to the truth. Wisdom of this kind does not come down from above but is earthly, unspiritual, demonic." James 3:13-18
First of all, doubt and unbelief are two all-together different realities. Doubt can be momentary, caused by temptation, overcome in faith--to the glory of God! Doubt can be the mind wrestling with the mystery of God--inspired in a new intellectual, and personal, thought-paradigm, where the Holy Spirit inspires us to see things in a more mature manner.
Unbelief, however, is the seed of the 'earthly, unspiritual, demonic' forces at play.
Unbelief comes from many purely human cults such as the cults of youth, personality, power, attractiveness, influence, talent, riches, pleasure, and the ideologies which promise utopia in scientism, secularism, progressivism, militant atheism, radical Islamism, and all 'isms' void of the One Loving God.
"The precepts of the LORD are right, rejoicing the heart; The command of the LORD is clear, enlightening the eye. The fear of the LORD is pure, enduring forever..." Psalm 19:8-10, 15
The Trillion dollar question is: 'What makes you joyous, see clearly, and live purely?'
If it is God, you are living faithfully, living within living faith; in effect, letting God be God. If joy (pleasure, and a sense of well being), and sight (personal vision), and purity (self-sufficiency), come from anything but God, be sure that you are entering the danger zone of unbelief.
For the world, and prideful people, and demons, all want your worship. As humans, there can only be one faith--as there is only One Source. Which will you choose...the God of faith, or the many gods of unbelief?
"'But if you can do anything, have compassion on us and help us.' Jesus said to him, ‘If you can!’ Everything is possible to one who has faith.'” Mark 9:14-29
Notice, above, that the man doubted both Jesus' power and his compassion! This is what the 'principalities and powers' of unbelief seek to spread...like a cancer. Through demythologization, Jesus is robbed of His Divinity. Through the hypocrisy of his disciples, Jesus is robbed of His compassion.
There's only one solution.
Catholics, and all believers in Jesus as both Son of God and Savior of man, must allow God's 'One, Holy, Catholic, and Apostolic' Church to re-evangelize their hearts to the Merciful Truth of Jesus Christ.
Re-evangelized in Baptism, confession, and Eucharist.
Then we must march forward, resolutely re-evangelizing the world, not only through Word and doctrine, but also through a reinvigorated spirit, reinvigorated in faith, hope, and charity, inspired in the Holy Spirit of belief.
Then, when we meet our brothers and sisters in the world, and they say: 'I do believe. Help my unbelief,' we will be the mustard seed that keeps them alive in Jesus!