Some Catholic authors in the past conjectured that the Apostles baptized with the formula, “I baptize you in the name of Jesus.” Thomas Aquinas gives that opinion in his Summa Theologiae, to name one example. And these were good Catholics who recognized the correctness of the Trinitarian baptismal formula and had no bones about the Trinity. My point is not to cast doubt on the Catholic teaching–the Church has ruled decidedly against it–but to illustrate how easy it is for people lacking guidance and firm grounding to fall into error.
Here’s a source that might be helpful. In the beginning of Book Four of the Summa Contra Gentiles, St. Thomas describes a pretty wide span of Trinitarian and Christological heresies, giving the heretics’ arguments and then refuting them from Scripture. Hope it might be helpful. Look particularly at the parts about Sabellius.
dhspriory.org/thomas/ContraGentiles4.htm
