Why to Catholics believe that their Church is the one true Church of Jesus Christ?

Wouldn't it be more reasonable to believe that Christ's true Church is a spiritual union of all Christian denomination?

Catholics believe that theirs is the one true Church of Jesus Christ, firstly, because theirs is the only Christian Church that goes back in history to the time of Christ.
Secondly, because theirs is the only Christian Church which possesses the invincible unity, the intrinsic holiness, the continual universality and the indisputable apostolicity which Christ said would distinguish His true Church .
Thirdly, because the Apostles and primitive Church Fathers, who certainly were members of Christ's true Church, all professed membership in the same Catholic Church (See Apostles' Creed and the Primitive Christian letters).
Wrote Ignatius of Antioch, illustrious Church Father of the first century: "Where the bishop is, there let the multitude of believers be, even as where Jesus is, there is the Catholic Church."
Our Lord said: "There shall be one fold and one Shepherd" ( John 10:16 ), there is only one Church and one door into it, and one Shepherd over it. Yet it is well know that the various Christian denominations cannot agree on what Christ actually taught. Since Christ roundly condemned inter-denominationalism, that house cannot stand." (Mark 3:24-25), Catholics cannot believe that He would even sanction it in His Church .

The Marks of the Church

In the Creed we profess the Church to be one holy catholic and apostolic.
These four characteristics, inseparably linked with each other, indicate essential features of the Church and her mission.
The Church does not possess them of herself; it is Christ who, through the Holy Spirit, makes His Church one, holy, catholic, and apostolic, and it is He who calls her to realize each of these qualities.

The Church Is One

John 17:11
Holy Father, keep them in your name that you have given me, so that they may be one just as we are.
John 17:21
... so that they may all be one, as you, Father, are in me and I in you, that they also may be in us, that the world may believe that you sent me.
Eph 4:4 - 5
There is one body and one Spirit just as you were called to the one hope that belongs to your call, one Lord, one faith, one baptism, one God and Father of us all.
1 Cor 10:17
Because there is one bread, we who are many are one body, for we all partake of the one bread.
John 10:16
(Jesus said) "there shall be one flock, one shepherd."

The Church is Holy

The Church of the apostles was holy. When we say that the Church is holy, we mean among other things that she had the all-holy God as her author.

Eph 5:25 - 27
Christ loved the church and gave himself up for her, that he might sanctify her, having cleansed her by the washing of water with the word, that he might present the church to himself in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish.
1 Tim 1:15
Christ Jesus came into the world to save sinners.

The Church is Catholic

The third great historic mark, or note, of the one true Church is that this Church is Catholic. Catholic means "universal." It refers as much to the fullness of the faith she possesses as to the undeniable extension in both time and space that has characterized her virtually from the beginning.
Mark 16:15
He said to them, "Go into the whole world and proclaim the gospel to every creature."
Col 1:5 - 6
The word of the truth ... in the whole world ... is bearing fruit and growing.

The Church is Apostolic

Finally, the Church that issued from the commission of Christ to the apostles was apostolic. Christ founded the Church upon the apostles.
John 6:70
"Did I not choose you, the twelve?" (Jesus) asked them.

Loving endlessly God , essence of Catholic Faith

To be a Catholic then is to desperately fall into the love with God.
Catholic Faith is to seek and respond to the love of God, to live out the love of God towards our neighbors including those closest to us, and to spread the love and peace to counter the evil influences of the world.
Catholic Faith is to fall in love with the Word of God in the Bible.
Catholic Faith is to love to, to long for, and to make efforts to receive Jesus present in the Holy Eucharist in the Eucharistic celebration.
Catholic Faith is to experience and to witness the true peace of God.
Catholic Faith is to be overflowed with joy and love of God in witnessing Catholic Faith in the family, at work, or at school.

Unconditional Love of God

Catholic Faith believes in the value and dignity of each individual person, including the weakest.
The love of God towards us is unconditional. Catholic Faith preaches the profound love and forgiveness of God, which we may better experience when we also forgive other people.
Although the fruit of Catholic Faith should include striving to perform acts of love, Catholic Faith is not a religion that only preaches justice and the need to do good deeds. Justice is already known to many people without Catholic Faith. The profound love and forgiveness of God surpasses even the justice of God. The coming of Jesus is not to bring justice but to reveal the unconditional Love of God.
Catholic Faith believes in the gifts of different talents to each individual in order that we may use them to serve others in our daily work at home and at work.
Catholic Faith believes in the love and healing from God especially when one has been deeply hurt by people in the world.
Catholic Faith believes in the use of God's gift of sex only within marriage in order to experience the love of God and to build up families with love.
Catholic Faith believes in the love of God present in us. However the lack of love and hurt from many people often makes it more difficult to believe in the love of God. It may take strong Faith to believe in and to rediscover the love of God.