to say continue is faith means faith comes from self. from out own power. and is based on ourself. And not based on the one we are having faith in.
that makes it a work.
A work is anything which comes from self. which is for self (ie, I am taking care of my own needs)
to say we must continue in faith is basically saying as long as the person I have faith in does what I want, I will trust him, but the second he stops. I will take my faith and put it in something that will take care of what I want (selfish desires, not actual needs, because God will always take care of our needs, He will not always give us what we desire, he knows us better than we know ourself. and he knows what is best)
so one must ask, if a person had this kind of faith to begin with, did they ever have faith in God? No. They just for an amount of time thought God would give them what they desire, so they decided to try this faith thing out. Once they realized God will not give them all they want, the reason for their faith ceased, and they fall away..
True repentance leads to true saving faith that is not self seeking, and thus will never fade away, Because it never seeks for self. it relys on God to know what is best for us, and may weaken, or vary, but will never fade away, because God NEVER does us wrong..
Only if we are selfish and seeking selfish desires will we ever stop trusting God because God never promised to give us everything we want,