Growth in establishing the Kingdom of God on earth is defined not so much by church membership among those who desire to love God with their whole heart, mind, soul and strength, and others as themselves, but in how we grow as individuals in Christian virtue.
A person who is catholic (where catholic is used in its original Greek sense, i.e. unified) seeks to build the Kingdom of God on earth through being a visible witness through charity.
“I give you a new commandment, that you love one another. Just as I have loved you, you also should love one another. By this everyone will know that you are my disciples, if you have love for one another.” –John 13:34-35
Thus, the extent that we show love for one another is the extent that we make the Kingdom of God visible on earth. However, sin, or vice, as opposed to virtue, sets itself against the practice of love towards God and one another. Therefore, how unified we are, is based on how we grow in Christian virtue, rather than membership in one denomination or another, or how well we know or adhere to the doctrinal tenets that define that denomination, except to the extent that those doctrines help us grow in virtue.
Similarly, the works or fruit that a person produces in pursuing Christian virtue and avoiding sin may be sweeter from one denomination to another, even though they adhere to doctrines another may consider erroneous, and vice versa. Nonetheless, the extent we are unified corresponds to how well we practice virtue as opposed to our unity through shared doctrine and more or less membership. The caveat to this is that some doctrines, and correspondingly, denominations, can foster greater virtue.
Consider for a moment the Sacrament of Penance, a.k.a. confession in the Roman Rite of the Catholic Church. If a person is struggling with a certain vice, say inappropriate thoughts, pornography and self-abuse, but does not seek help from another, the likelihood that they’ll continue struggling is higher than someone who holds themselves accountable through another. If the vice causes them shame, the fact that they’re accountable for the activity to another helps them overcome it. How often do inappropriate fantasies and self-abuse ultimately lead to fornication? How likely is someone with self-control going to be faithful in marriage versus someone who has not developed it?
Also, consider the power of reciting the rosary in strengthening belief in God’s work throughout salvation history, in heaven, and for men who struggle with seeing the personhood and dignity of woman, which is not a difficult thing to struggle with in a culture permeated by an attitude of sexual permissiveness. In light of these things, how much more likely will we be in establishing a culture of life, where the birth rate far exceeds the death rate, where programs like Medicare and Social Security can continue working?
Now church history suggests that unity of doctrine within the body of believers, from a doctrinal standpoint, is often unrealized. One would think that those doctrines that foster virtue are readily embraced over others that don’t, thus helping to unify the Body of Christ. Sadly, we’re often more concerned with how happy we are and how well we’re integrating into a new community, than with how well the doctrines embodied in the community help us to grow, especially since such growth can involve suffering. However, it is still the fruit and good works that we produce as believers that are either sweet or rotten; the doctrines of our denomination, in most cases, do not determine this, except to the extent we live them out and they are more efficient in fostering growth in Christian virtue.
Note that the first universal evangelical call is,
“The time is fulfilled, and the kingdom of God has come near; repent, and believe in the good news.” –Mark 1:15
Before there can be growth in virtue, there must be repentance. Before growth in belief, repentance. And, since it is impossible to please God without faith, it is impossible to approach Him without a contrite spirit, with humility and repentance. This is our part, our leg work, in entering into salvation.
Vice and sin cloud our judgment, so much so that we cannot believe in the Gospel. In this sense, the gift of faith is truly a gift; it is not something we can choose like we might choose a flavor of ice cream, except, perhaps, to the extent that we are repentant, that we turn out hearts toward God and pursue righteousness and piety. Of course, this involves knowing Him, for you cannot abide with someone you love if you don’t know them. Thus, our desire to love Him and others as He has loved us, according to His call, determines our sweetness and our unity within His body.
As plants grow towards light, we should always gravitate towards a greater understanding of truth and the beatific vision. Albeit, God works with each of us uniquely, given our differences of family upbringing, ethnicity, environment, genetics and so forth. This contributes to the mystery that allows us to practice charity and humility, right now, being unified in His body, in spite of doctrinal differences.