There exists a being, whom we call God. God is a trinity of three Persons who are intimately bound in a union of pure love. Living in this bond of love, they are (1) infinitely happy, and (2) perfectly united in the one Godhead. That is, the Three are truly One; God is one. This is the power of love. We name these three Persons: the Father, the Son, and the Holy Spirit.
God is an alternative reality from our point of view: God is outside of time and space. God is pure, intrinsic life, which we call spirit. God's life is a state of love between the three Divine Persons.
God is a necessary existence - God must exist. Nothing else causes God to exist; God is non-contingent. God created all things; all things are contingent upon God. God created all things with complete freedom; it was God's choice to create and to create the things that are.
Humans are animals (primates) with an added spiritual nature; we call the spiritual nature of each person the soul. We have faculties in both the natural world and in the supernatural world. There exists a convergence between the natural (animal) faculties and the spiritual faculties. In a perfect world, our spiritual faculties would control our animal faculties.
We have a spiritual nature because God wants us to be united with the Divine nature, and enjoy the life of perfect happiness in Divine love. God unconditionally loves us; if we love God back, we will experience this happiness. We will be united with the Godhead, while retaining our individualism.
3. The Problem
The problem is simply that we have an innate tendency to self-love, which is a roadblock to loving God. We are not capable of opening up to God's love and loving another as we ought. Our animal faculties tend to dominate our spiritual faculties, even if we don't want that to happen. We take from others, instead of giving to others. Our love for others is tainted by selfishness - a desire to get something from another as a gift to self, rather than pure self-giving. We call this state of affairs Original Sin, humankind's fallen condition.
This fallen condition began at the very start of humankind, with the very first humans. Every human is conceived in this fallen state (with the historic exceptions of Jesus and His mother Mary; they were exempted by a special act of God).
No human being has any ability to overcome this fallen state by some ability or action. As such, humanity cannot find union with God, therefore cannot experience perfect happiness.
4. The Solution
God resolved this situation for us. God the Son became a man: Jesus. Jesus was both God and man united, both natures distinct yet united in the one Person. Jesus lived and died as a man. He then rose from the dead by His own Divine power. Through His act of death and resurrection, Jesus resolved the problem of our fallen state - we call this Redemption.
Ordinarily, if we have faith in Jesus as God and live according to His directions, we will be transformed into a new kind of creature, able to find union with God and infinite happiness in a bond of unifying love with the Most Holy Trinity forever.
To have faith in Jesus means to believe what He taught, to understand that what He taught was true. Our sources for such truths are Sacred Scripture (the bible) and Sacred Tradition (the Sacred Liturgy and the Apostolic teachings handed on).
To live according to Jesus' direction means to frequently commune with God (prayer), to enjoin ourselves to His Mystical Body (the Church) and its life (the Sacraments), and to form habitual behaviors directed towards self-giving instead of taking (morality).
We call this way of faith and life the spiritual life. By living in this way, God gives Divine Life to our souls, enabling us to levels of self-giving not possible by nature. We call this gift of God's own life grace. We call the act of God giving us this grace mercy.
We want to be happy, not unhappy. By means of God's grace, we can be perfectly happy. In theory, one can achieve perfect happiness through grace immediately. We know, however, through centuries of experience by countless Christians, that the process of activating grace is normally a lifetime process, with bouts of unhappiness along the way. This is, however, the only way and Christians give themselves completely over to this way of life, which changes us, transforms us into something new.
This is the distilled Catholic Faith. Everything else is details of the above.
"His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire." (2 Pet 1:3-4)