Quote from Boice’s Expositional Commentary on Ephesians:
“The second meaning of the word “faithful” is “to continue in faith” or, as we might say, “to keep the faith.” It involves the idea of perseverance in the Christian life, enduring to the end. Jesus said, “He who stands firm to the end will be saved” (Matt. 10:22). Usually, when Reformed Christians talk about the perseverance of the saints, they mean the perseverance of God with his saints. They wish to say that the only reason why any of us are ever able to stand firm to the end is that God is faithful to us. Well and good! But it is also true that precisely because God perseveres with us, we also must persevere. We must be faithful. It is therefore also proper to say that a Christian is one who is characterized by a full faith to the very end of life.”
James Montgomery Boice, Ephesians: An Expositional Commentary (Grand Rapids, MI: Ministry Resources Library, 1988), 6.