Some of the language Milton uses is prone to misunderstanding, which affects readers' ability to recognize and grasp the themes and main idea that he is communicating. Some of this confusing language is discussed below.
PATIENCE: "Patience" used here is not a personal trait, rather a Fruit of the Spirit granted (given) by the indwelling of God's Holy Spirit. The poet affectionately ("fondly") queries God (line 7), and one of the Fruits of the Spirit, personified, answers back. Thus the answer to the important query Milton asks is given objectively from God's Spirit not subjectively from the poet's self.
PREVENT MURMUR: A "murmur" is a complaint, "a private expression of discontent." Asking a question ("Doth God exact day-labour, light denied?") and complaining cannot be equated: they are different. Patience answers the poet so that a lingering, unanswered query does not turn to a complaint of discontent: Patience replies to prevent the upwelling of "that" potential murmur.
PATIENCE REPLIES: A dialogue develops between the poet and the Fruit of the Holy Spirit, "patience." Here patience is personified as "Patience" and given human characteristics, including the power of discourse. "Patience" is neither a human trait nor one of the Seven Virtues but rather the gift of God's indwelling Holy Spirit (Galatians 5:22-25). It is important to understand that when Patience speaks, it is not a trait of the poet's personality or faith nor is it a virtue exercised by the poet that is speaking: the dialogue is not an internal subjective one. Rather the dialogue is an objective one carried out between an agent--a fruit--of the Spirit of God, the Holy Spirit, and the poet. This gives credence and universality to the answer the poet receives to his question: The answer is that God provides and accepts alternative service to that service which can no longer be given.
MILD YOKE: A "yoke"--a device borne upon the necks of oxen which are used to do labor--signifies constraints in direction, resources, labors: the oxen go where they are constrained to go and do what their constraints permit them to do. Patience says that God's constraints ("yoke") are "mild" although we feel their weight, such as the weight of blindness, and although they constrain our directions, resources and labors.