Many of them believe that unless one is a member of the Catholic Church, one is not really a Christian. My response to that would be Matthew 18:20:
I believe that Jesus is the son of God and I follow His teachings. Does this mean I will be welcomed in heaven? We’ll see.
Your witness is very kind and welcome indeed. Remember however what Christ did: he founded a Church. And the apostles said: there is one bread, one body, one Church, of which we are all members and which is the mystical body of Christ, who is Head of the Church. The fact is that each and every validly baptized Christian is a member in partial communion of the One, Holy, Catholic, and Apostolic Church, which currently subsists in the “Catholic Church” governed by the successor of Peter and the bishops in communion with him.
One must realize that each and every Christian community derives from the Catholic Church

Surely one may find some of her teachings significantly hard, and the temptation to choose which teachings to follow and which to discard has always been present, even since the days of the Apostles (as we clearly read in the New Testament) and of the Church Fathers. The point is that the Church simply passes down the apostolic teaching, the deposit of faith, as she received it. Nothing novel is introduced, nothing is removed. The deposit of faith obviously grows, because “
the kingdom of God is like a mustard seed” which begins very small and grows very much in time, and because to the Apostles Christ said:
"there is still much that I must tell you, but you cannot bear it yet…the Holy Spirit will take what is mine and reveal it to you", and he added:
"those who listen to you, listen to me - but those who reject you, reject me". Christ adds nothing to His teaching, but never ceases to talk to us and to guide the Church into a greater understanding of the Word, through the apostles and their successors - to them in fact He said: “
The mysteries of the kingdom of heaven are revealed to you, but they are not revealed to them; so that seeing they may not see”. Christ established a Church, visible and perpetual, with a hierarchy of workers: the faithfuls, the deacons, the presbyters, the elders (‘bishops’), and the chief of the bishops, successor of Peter, His Vicar, to whom He said: “feed my lamb, take care of my sheep”, all the while He remains the invisible Head of the Church.
Christ is clearly next to everyone who calls His name, and God does not reject even those who do not know Christ, not even those who profess to not believe in God (where they call “God” whatever false idol they have constructed in their own mind, pretty much like those who reject the “Church” when they call “Church” whatever idea they themselves came up with, which is obviously not that of Mother of us all and Spouse of Christ).
However, Christ did give His Bride, the Church, some nuptial gifts…the Sacraments, the ordinary means of sanctification…and they are so beautiful…I was myself a Christian for over 10 years - and one who didn’t quite like the Church too much - before my conversion. I realized after receiving the Sacrament of Reconciliation and kneeling before the Blessed Sacrament, the Eucharist, that I did not know Christ…that I had heard of this Christ, but I had never met Him…that I heard how God forgave sins, but I had never experienced His forgiveness…I read of this God-man that calls us friends, but I never felt as His friends, but rather as a sinner much afraid of His judgment…I read that He gave us the right to become children of God, but I had never felt in my heart the Spirit crying: “Abba’, dad!”. I heard of the gifts of the Holy Spirit, but I knew nothing of them until after Confirmation. I read about true love, true chastity, true liberty from sin, peace in tribulation, serving only one master…but I never truly loved, was never truly chaste, was never free from attachment to sin, never had peace in tribulation, and always served two masters, often despising the light and embracing the darkness. The Church became my home, Confession my personal encounter with Christ, the Mass the moment in which the mystical union of my poor soul with the Divine Bridegroom was consummated. I had always seen the image of the fire coming out of the Sacred Heart of Christ…I had always seen the image of the fire that embraces the blessed seraphim before the throne of God…but I had never felt it in my heart and around my soul…and the day I did, when I realized that in the Church I had found Christ and had fallen in love with Him…that day I realized what the Church was, and that there was no rule to harsh or difficult for me not to give full assent of will and mind…out of love, all things are possible, and we long to drink of the same chalice He drank from…to experience on our shoulder the Cross and on our head the Crown of Thorns, until the blessed day when we will be transformed in the blink of an eye, and see Him face to face…
…I pray that God may keep you and all always in His Sacred Heart, tight between it and the thorns that surround it, where none may touch you…but how much simpler would the Via Crucis be if you walked closer to the Church…!