What is the difference between faith and opinion?
The difference between faith and opinion consists in the fact that, in the case of faith--human or supernatural--, the will moves the understanding to assent with certitude, without fear of the opposite position being true, based upon the testimony and credibile authority of another.
Summa Theologica, II-II, q. 1, a. 4; De Veritate, q. 14, a.1.
Do not confuse faith with simple belief which in common language is practically the same as opinion. We say, for example: "I believe Martha has gone out, but I am not sure"; where "I believe" means "I opine" or "it appears to me".
Faith and Evidence
With faith there is no--as there is with opinion--fear of being mistaken. Therefore, from the point of view of its firmness, faith is a type of certitude. For certitude may be certitude of evidence--based upon the objective manifestation of the truth--or certitude of faith--which is based on the authority of the witness, manifested by the evidence of his credibility. Regarding the reason of knowledge, the certitude of evidence is always more perfect. But the certitude of faith--regardless of the obscurity of the knowledge--can be more perfect regarding the firmness of adherence.
Alejandro Llano, Gnoseologia. Pamplona: Eunsa, p.63 (My translation from the Spanish).