Well, you are a Christian.
That has not changed.
Now however you are a Christian in full communion with the Church of Christ.
This answer they would not understand or agree with.
So you can simply tell them that you are now a Christian in the Catholic Church.
And you do not have to argue with them, nor answer their objections, nor try to convince them that it was a good choice, nor try to convince them to follow you.
You have to simply be yourself.
When St. Paul received light and sight from Christ, he simply became a member of the Church. He did not bother to explain it, neither to the Church, nor to his previous brothers in the faith. He was simply himself, the same Paul of old.
Some of his friends greatly pondered what had taken place - and it is reasonable that some of them may have converted as well - others simply didn’t care, others hated him. That does not make the least of a difference. Ultimately, we are who we are in the eyes of the living God.
In fact, the sign of the Christian is profound peace amidst tribulation. The greater your serenity in sharing the good news to them, and the greater your serenity in the face of their probably quite upset reaction, the more you shall reflect the mark that Christ has placed on your heart.
Do you think it is shocking for a protestant friend to find out that you are joining the Catholic Church - as if their doctrine and everything they say or do was not based on 15 centuries of Catholic Church? How much more was this shocking for those early Christians who were Jewish? How much more was this shocking for those early Christians who were pagans? The risk they took was more than to make their friends upset or to lose their friendship…it was their life what was at stake. We often think that martyrs are a thing of the past…they are not…just like there is exterior and interior mortification, there is exterior and interior martyrdom…and a Christian is always ready to die to himself for the love of Christ.
Remember that Christ wants all to be united in one body with one shepherd. Your witness is for the greater glory of the Lord. It will make them ponder. It will make them think. Perhaps it will make them curious about this Church they dislike or hate - probably with no real reason. When I was not a fully initiated Christian and far away from the Church, I thought it was quite natural to dislike the Catholic Church, even to be angry at her. I had no good reason, except, of course, the usual worldly reasons. Only when I opened my eyes and looked at Her, I realized that I did not know what I was talking about. I heard, though not with my ears, the great question: “Saul, why do you persecute me?”
So do not worry about having to defend yourself…you don’t! It is them who are called to defend themselves, though they may not know it. Remember when Pilate and Herod interrogated the Lord, and He replied nothing? Who was the judge? Who was the ruler? Who was the king? The man with the scepter and crown, or the man bound in chains? Now you are more and more aware about how great a Tradition is found in this Church that is “the world’s largest, and Christianity’s oldest, religious body…a colossal success and a colossal enigma”. Consider the humbleness of the Lord’s mother, that sweet “lowly handmaid of the Lord”…realizing the wonderful and mysterious things that were engulfing her, she simply bowed her head and treasured all in her heart…not fearing her parents, or Joseph, or her community…simply trusting that whatever God had deigned to begin in her, He would bring to completion for His greater glory, for her own good, and for the benefit of souls. To Elizabeth calling her the mother of God, she simply replied: “my soul magnifies the Lord…my spirit rejoices in God, my savior…for he has seen the lowliness of His handmaid…he who is mighty has done great things for me…”
