Much conversation about sport is about perseverance, training and improvement. To keep going when the training is hard is in itself quite an achievement and to improve all the time takes real commitment and hard work. The Bible, in the Letter to the Hebrews, draws upon the analogy of running the race and expresses something of the doggedness of keeping going. It is a well understood. Faith is like running a race and about keeping going.
Those first followers of Jesus were inspired by the Holy Spirit and their enthusiasm was high but I suspect there were times when all was not so well and they were tired and struggling. It is the same for every generation of disciples: some are really enthusiastic and they set off at a great pace but it dies out very quickly as they are confronted by disappointment; some are a bit more steady and they seem to be going along well but there is no substance, no reserves upon which to draw strength and there are those whose faith is priceless, rooted firmly in a sound base and they go on and grow beautifully.
Enthusiasm for any sport or hobby is incredibly important and sometimes very difficult. Faith can be likewise. When life is not so good, or other things press in upon you or when something drastic happens and you feel overwhelmed then I suspect faith flags. The Christian is called to keep going in the race knowing that at the end they will all be victors.