The Church certainly is an instrument in God's hands for the salvation of the world. It is God's Church. He has made it according to his plan and for his purposes. That the Church should be one and visibly one is certainly part of the founder's will and intention. The visible unity belongs to the nature of the Church and is very important - a top priority.
Visible unity demands a visible center of unity.
For the local community there is the parish priest. For the diocese the bishop. For the patriarchate there is the patriarch. And for the whole fold, for the universal Church, there is the Pope. That there should be a pope is part of Christ's plans for his Church.