May we answer your questions?
Are Catholics considered Christian?
YES! The Catholic Church is the original Christian Church, established by Jesus Christ Himself over 2,000 years ago, and handed on to His Apostle Peter (our first Pope). This can be clearly seen in the Bible as well; Jesus said “And so I say to you, you are Peter, and upon this rock I will build my church, and the gates of the netherworld shall not prevail against it. I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven.” – Matthew 16:18-19
Jesus gave His authority to Peter to oversee His church (singular) on earth, and that same authority has been passed on to each Catholic Pope (successor of Peter) without any gaps up to the present day.
Why shouldn’t I receive communion at the Mass if I’m not Catholic?
First, we need to be clear that the Catholic Church desires the entire world to receive the Eucharist (Communion) as often as possible. However it’s out of love that the Eucharist is not recommended for both non-Catholics, and even for Catholics that are not in a state of grace. This is because of the gravity of these words from 1 Corinthians 11:27-28 “Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup. For anyone who eats and drinks without discerning the body, eats and drinks judgment on himself.”
As Catholics we believe that we’re receiving the true physical body, blood, soul, and divinity of Christ, not just bread and wine as a mere symbol of the last supper. During the Mass, the bread and wine on the altar are transubstantiated into the actual body and blood of the Lord during the prayer of consecration, where the priest recites the words of Jesus at the last supper… this IS my body… this IS my blood. (not just a symbol of it)
Lastly, the importance of receiving the Eucharist (communion) can be seen in the words of Jesus “Truly, truly, I say to you, unless you EAT THE FLESH of the Son of man and DRINK HIS BLOOD, you have NO life in you; he who eats MY FLESH and drinks MY BLOOD has eternal life, and I will raise him up at the last day. For MY FLESH IS REAL FOOD, and MY BLOOD IS REAL DRINK. He who eats MY FLESH and drinks MY BLOOD abides in me, and I in him. As the living Father sent me, and I live because of the Father, so HE WHO EATS ME WILL LIVE BECAUSE OF ME. This is the bread which came down from heaven, not such as the fathers ate and died; HE WHO EATS THIS BREAD WILL LIVE FOREVER” (John 6:53–58)
Because a non-Catholic will likely have varying beliefs about Communion being only a symbol, or not understanding the Eucharist to be the true body and blood of Christ, as well as them potentially having mortal sin that has not been forgiven through the Sacrament of Confession, we recommend them not receiving Communion so as to not eat and drink judgment upon themselves. As you can see this is out of pure love, not elitism or an attempt to create division.