Q. Why are the Apostles’ Creed and the Nicene Creed different?
A. Like all Catholic creeds, the Apostles’ and the Nicene provide something of a summary of the faith that we profess. Nevertheless, no brief creed can include every single fundamental Christian belief, and specific historical circumstances shaped which of the fundamentals were included or emphasized in each creed. The result is some variation in content, though of course not in doctrine.
Here’s an example of a fundamental tenet of faith that is not expressly stated in either creed: The Nicene notes that Christ “rose again in fulfillment of the Scriptures,” and that the Holy Spirit “has spoken through the prophets.” But it doesn’t contain, nor does the Apostles’ Creed, an explicit statement of the divine inspiration of Scripture.
Why not? Largely because, in the period when these creeds were developed, there was no serious challenge being raised by heretical factions to the truth that Scripture is divinely inspired (though there were of course heretical challenges to the Church’s choice of books to be included in the Bible).
On the other hand, the Nicene Creed (technically, the Niceno-Constantinopolitan Creed) had its origins in the Church’s response to an ancient heretical movement (Arianism), which taught that God the Son was a creature made by God the Father, and thus was not fully equal with Him. That’s why this creed emphasizes and elaborates on the full divinity of Jesus Christ: He is “the Only Begotten Son of God, born of the Father before all ages. God from God, Light from Light, True God from True God, begotten, not made, consubstantial with the Father; through Him [that is, through God the Son] all things were made.”
On the other hand, the Apostles’ Creed, having developed under different historical circumstances, speaks of Him simply as “Jesus Christ, His [God the Father’s] only Son, our Lord.”