This verse is packed with ideas which are brought together in a single clause, as evidenced by the number of prepositions: ‘by power’, ‘through faith’, ‘unto salvation’, ‘in the last time’. Peter identifies the agent of God’s power, the instrumentality of faith, the outcome of salvation, and the proximity of that outcome.
Believers are kept and it is vital that they are, for their struggle is against opponents that are too strong for them, whom they could not hope to defeat if left to struggle alone. The devil is intent on the destruction of every single believer and he goes about his work with relentless persistence. The fact that he has never yet once succeeded in destroying a true child of God does not in any way discourage him for he is driven by a hatred so intense that it will never let him rest. And he possesses powers which dwarf the power of man so that the most brilliant of human beings that has ever lived could not defeat him in single combat. Left to ourselves we would be overcome by his subtlety and deceitfulness.
Through faith he will keep you. Does that mean that faith can never fail? No, faith can fall, so what then has happened to the power of God? Faith can fall because of our sin and our foolishness but the power of God will one day re-vitalise it. He may have to discipline us very firmly. He may have to mete out some chastisement, to bring us to our senses, but he will do it and he will restore our faith. It is a sad thing to become backsliders, to allow faith to wither. How terrible a thing to lose touch with our Saviour and with the blessing and the power of God, even for a season!
It starts when you neglect your daily prayer. A day or two or three or four and you are on dangerous ground. Or you stop reflecting. You reflect on all sorts of things but you stop reflecting on Christ and his goodness to you and all that he has done and reading his word. When you fail him, perhaps you begin to lose your temper. Or you begin to think of yourself all the time. You begin to think of your possessions and what you have got and what you want and these things fill your horizons.