Textual: It is tempting to see Mormon excerpting a phrase from a record of Alma. When Mormon states that Alma commanded that "there should be no contention one with another, but that they should look forward with one eye, having one faith and one baptism, having their hearts knit together in unity and in love one towards another" this is either Mormon's synopsis of the intent of Alma's teaching, or a direct quotation. With Mormon's clear (and deserved) admiration for Alma, one would suspect that Mormon took the opportunity to insert some of Alma's words here. It is most likely a referenced citation, one that says what Alma said, not one that inserts the precise wording.
Of course the "one faith, one baptism" phrase also echoes Ephesians 4:5: "One Lord, one faith, one baptism." We cannot tell whether or not this particular phrasing in the Book of Mormon owes a debt to Joseph's Smith understanding of Paul, but the sentiment of unity through the community of baptism is completely at home in Alma's context. Even should the specific wording owe a debt to Paul, the meaning is Alma's, and describes Alma's creation of this new community.
Scriptural: Mormon indicates that through the unity of their faith and baptism that the people became "children of God." We cannot tell whether or not this is a phrase that was original to Alma, or added by Mormon. The nature of this part of the text suggests that it is Mormon's phrase, his synopsis of the conversion process the people underwent. If that is correct, then we may see in this Mormon's understanding that the people of Alma have undergone the great change that Benjamin describes in his roughly contemporary sermon in Zarahemla:
"And now, because of the covenant which ye have made ye shall be called the children of Christ, his sons, and his daughters; for behold, this day he hath spiritually begotten you; for ye say that your hearts are changed through faith on his name; therefore, ye are born of him and have become his sons and his daughters." (Mosiah 5:7)