Funny question. I'd say: first came the sound /erik/. Second, in really ancient times, old Norse was written using runes, not latin letters. There is no difference between the c (pronounced k) and k in runes. Only than came latin letters, and Nordic languages consistently use the letter "k" for the sound "k". The c-spelling is French and English (influenced be French).
Also be prepared for huge spelling variation when you look at old documents. The was no such thing like standardised orthography for a very long time in history.