Here’s how I like to think of it:
We know that Christ is Body, Blood, Soul, and Divinity in the Eucharist which we call Communion. We know the Church is called the Body of Christ, and the Holy Spirit is its Soul.
Well, other Christians and even people of other faith unknowingly through implicit desire and God’s grace can be within the invisible bounds of the Church, even though to be in full and perfect communion you have to be part of the visible, institutional Church and submit to it.
It’s like this:
Catholics are united to the Divinity aspect of the “body of Christ” (his Church) in baptism through sanctifying grace, and the Soul of the Church by the outpouring of the Spirit in baptism, and the perfection of this in Confirmation. They are united to his body and blood through the Eucharist, and if they are faithfully united to the Pope, they are like well functioning cells in the Body, even if they are still somewhat diseased by sin. If they mortal sin or especially if they are excommunicated, it is like they are a diseased tissue that has been amputated. It is a grave sin to recieve communion in this state knowingly because it is unnatural and a lie; it is uniting oneself to the Mystical Body of Christ in Communion without being united at the time to his Soul and Divinity by the Holy Spirit and Sanctifying Grace.
The Eastern Orthodox are in closest communion as they have valid sacraments and eucharist, but do not submit to the Pope. They are like cells in a part of the body that is affected with a nervous twitch or something. They are not fully in synch with the holisticness of the body, but are close.
Protestants with valid trinitarian baptism are united to the Soul of the Church and its Divinity in sanctifying grace…but are not united to the body. This is unfortunate as it is unnatural for the soul to be disconnected from the visible body…nevertheless, they may be saved by their baptism if they are ignorant of the fact that they are rebelling against the unity of the True Church.
Finally, non-baptized people, by way of explicit desire…or even through invincible ignorance and implicit desire if all options for doing things the normative way have run out (ie, at the moment of death), may still be open to God’s grace, which he offers to all. These are united to the Divinity. However, human nature united to the Divinity is supposed to be manifested and incarnated as the human Body and Soul of Christ. It is God’s ultimate desire for these people not just to be united to him in grace, but also visibly as part of Christ’s united body, blood, soul, and divinity in the Church…which for these people may happen on the Last Day.
So, it is possible to be united to the Body of Christ in varying degrees…but it is not natural or ideal. Ultimately, God wants human nature to be united to him in the well-functioning communion of the Body, Blood, Soul, and Divinity of Christ…not just to be attatched to have one’s spirit attatched to only the divinity through grace, like the angels, or be united to the soul but not the body, for this tares Christ’s mystical bride apart unnaturally…