Then perhaps you would like to list the various sects around in the early centuries for us, and note how "orthodox" they are relative to your own faith today, nevermind the Catholic faith.
ALL of these "other Christians" were heretics whom you would disavow any common link to in a heart beat. Or at least you should! Whether you like it or not, your patrimonial link to the Apostolic Era runs through Rome. Regardless of whatever denomination you belong to, as a Protestant, you cannot go back in time before 1517 and link with earlier times. It simply cannot be done. As "bad" as you might want to make Catholicism out to be, it IS your Christian heritage. The multitude of Protestant denominations have since deviated from it to a greater or lesser extent, but it IS your ancestor in the Faith.
Since Catholicism can demonstrate commonality with these early Church Fathers, and all of their rival, contemporary "Christian" sects have effectively become extinct long since, a strong case can be made, based, among other things, on Divine Providence as well as logic, that the ONLY group of people demonstrating an unbroken continuum from the Apostles to now being the Catholics, THAT Church is likely to be the one Christ founded. Again, if this is not so, then the early Christians evidently got it all entirely "wrong" from the very beginning, and STAYED wrong for over 1500 years! The burden of proof on you would be to show how this is tenable given that the Church was founded by God Himself, and He promised to be with the Church ALL DAYS until the end of time (Matthew 28:20). None of the other groups that coexisted with the Catholic Church back then could trace their ancestry back to the Apostles, and, in any case, none of them even exists today. Therefore, not one of them can be the True Church. Nevertheless, I'll humor you, let's suppose any given one of them DID survive to the present: name *one* group, from the first to fifth centuries, that you would be proud to link yourself to as a Christian. If you cannot come up with one, and you insist that the Catholic Church, exemplified by the Fathers, is not that Church either, then you leave a 1500 year VOID where Jesus Christ, the omniscient, omnipotent Son of the Living God, FAILED to keep His promises regarding the Church's preservation! If He so miserably failed, then Christianity itself in any form is manifestly a FALSE religion, and both of us are wasting our time.
"the catholic church was not the only group of practicing Christians."
That's right! There were the Gnostics and others.