In my most recent post, How to Find Communion in a Church that Doesn't Care, I touch on something that has a more current application than is at first apparent - and that is this: the push to allow divorced and "remarried" Catholics to be allowed to receive communion is not an argument on the nature of marriage as it is an argument on the nature of communion.
To put it as simply as possible: our salvation, and all the good that we can do, is rooted in Sacrificial Love, which is Jesus Christ and the mystery of His mission, a mission of loving beyond the limits, which entails suffering, death and rebirth - a mystery in which we participate in so far as we are in communion with the Body of Christ, which is the Church. Participation in this mystery is a lifelong process of sanctification, which is the essence of eternal life. There is only one Baptism, one Lord and one Church, and therefore every single human on this planet finds himself or herself in some sort of relation to this Mystical Body, which is nothing less than the ongoing incarnation of God in time and history. How and to what extent we relate to this Body may be called "communion" - atheists being outside of communion with Christ at least visibly (though in their search for Truth and their good works they are in partial communion with Him, without their realizing or admitting it), and baptized Catholics who are free of mortal sin being in full communion with Him visibly (or in communion with Him as fully as they possibly can be at that moment). And this communion is a reality that is both conveyed and represented by the sacrament of "Communion", for, at the Eucharist, we share in the sacrifice, the spiritual and physical infusion of His precious body and blood into our own, a grace that remakes us and somehow changes our very nature.
But, as I say, all of us vary in the extent to which we are in communion with Christ. Even those of us who are most serious about our Faith, and who long for God as a hart longs for water (Ps. 42:1) are never in this life fully and perfectly in communion with Him, for to be in full and perfect communion with Him means to love as He loved - perfectly, selflessly, without sin or guile - and none of us ever seems to achieve that in this life, except perhaps the saints, and even they were also sinners who were busy repenting (see below).
Therefore, a married Catholic who has abandoned his or her spouse and is civilly "remarried" to another person, with whom this "remarried" Catholic is setting up house and having sex, is not by that very act in full communion with Christ. The first step toward communion is repentance. The first words of Jesus recorded in the Gospel of Mark are simply ...
"The time is fulfilled and the kingdom of God is near. Repent and believe the gospel!" (Mark 1:15)
... and the entire point of Peter's first sermon, his stirring speech on the day of Pentecost echoes this ...
“Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit." (Acts 2:38)
So, no matter what the Church decides to do as a disciplinary matter regarding divorced and "remarried" Catholics, they cannot change the definition of Communion. They can formally decide to admit unrepentant D&R Catholics to Communion, as they have informally decided to admit unrepentant pro-abortion Catholics to Communion, regardless of how this demoralizes and confuses the faithful. But such unrepentant sinners are simply not in full communion with Christ, and therefore in receiving Communion at Mass they are living a lie, to the disheartening perplexity of others as well as to the eternal peril of their own souls (see 1 Cor. 11:29).
For the authority of the Church extends to two arenas, the temporal and worldly one and the eternal and spiritual one. The Church is protected from error in exercising its authority in teaching on the spiritual realm, in matters of Faith and Morals. But the Church has always erred, at least to some extent, in exercising its authority in temporal and worldly matters, as it very well might err in formally deciding to forego the call to Repentance for those who seek Communion.
There is a natural quality of the Church and of all Christians that is worldly and sinful, and hence is part of the "long defeat" of history that Tolkien describes, while the supernatural quality is eternal and shows forth "glimpses of final victory".
Full communion with Christ is that final victory. All parodies of that are part of the "long defeat".