This is a very complicated one but I’ll do my best.
“He who eats the Body of Christ unworthily eats condemnation on himself”
(sorry slightly mis-quoted I think)
It is a grave sin to Receive the Eucharist when in a state of Mortal Sin.
There are specific sins which incur automatic de-facto excommunication the moment they are committed. These include procuring an abortion for yourself or some else, or assisting or instructing someone to do so. Depending on circumstances the lifting of that excommunication may be reserved to your bishop or even to Rome.
To reject a church teaching can have a number of consequences.
- if you know you are rejecting a church teaching, and reject it willingly then you are in a state of heresy. As such you are in a state of grave and possibly mortal sin. you should not be receiving Communion.
- It is however possible to be in a state of “Invincible ignorance” - you are not guilty because you hold your beliefs in good faith, and are not aware that you are rejecting a mandatory teaching of the church.
- It is possible that you are not guilty because you do not give deliberate and conscious ascent to being in a state of heresy - you believe your position is right, but would be open to having the Truth explained to you if someone was willing to take the time and able to explain the matter to you in a way you could understand.
I spent years in this third category regarding Marian Theology. I wanted to understand the teaching of the Church but couldn’t, and it conflicted with my understanding of Theology and revealed Truth.
The use of birth control methods and the use of Abortion, (ignoring for now the more controversial possibilities of how some birth control methods work by abortive means) are potentially in 2 separate levels of moral seriousness.
The use of birth control inside a marriage is a sin against the virtue of Chastity, and a refusal to cooperate in Gods Creation. It is a refusal to obey the first commandment given to Adam and Eve “Go Forth and Multiply”.
The acceptance of Abortion is more serious. it is a rejection of the commandment “Thou shall not Kill”.
The person being killed is in the most vulnerable and innocent category of human life, and there is no excuse or mitigation for this evil.
These are not optional or secondary doctrines. If you do not assent to these doctrines you are rejecting God. Thereby you are in a seriously dangerous position. How is it possible for you to enter into his presence if you are telling God that He is wrong and you wont accept his Revealed Truth?
You would have very little chance of being able to accept his salvation if you came face to face with him.
In terms of recieving Communion in this situation: think of this a bit like having marital relations with your wife:
If you are rejecting your wedding vows, and refuse to be properly reconsiled with your wife… outside the home you are going around calling her a slut, a liar and a cheat, but still come home and expect to consumate your marriage you are not only sinning against her in the actions outside the home you are doing it again at home and hurting her more by having those relations with her under false pretences.
The marital act should be “Making Love” that is not possible if you do not have love for that person. it is using them. - another sin again.
Recieving the Eucharist when in a state of mortal sin (rejection of God) is very like this but on a spiritual level. it is "Making Love " to God, while calling him and his Church a liar and a cheat when you are not in his “Bedroom” (the chapel).