Faithful is a beautiful word which nobody remembers. we must be faithful to people who help us when we are in odd [critical circumsance] time. till our last breath we should not forget it. otherwise we will be punished somehow or other. first to god given us a human form [janma], our parents to have given birth, bringing up us, giving good education, a place to dwell, good norma food, etc, then elderly people, then to our boss, then to others who r all helping us in any way.
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A god loving person will be faithful to God. Here faithful means complete confidence in the beneficial acts of God on his life. Faithful among pairs means again 100 percent confidence that each will think and act to improve their relationship and life's journey. Faithful for a religious man is his faith in the power of his religion to provide salvation for him. Hope by these three different situations I explained what I think as probable answer. (You can easily get the Google / Wiki answer, if needed)