The Sincerity of the Prophet Muhammad explored

We know that the Prophet (peace be upon him) was a sincere person. Almost everyone who has studied the life of the Prophet Muhammad (peace be upon him) would non-hesitantly admit that the Prophet (peace be upon him) was sincere. Regardless of whether he was sincerely right or wrong or whether any another aspects of his character could be criticized, they would admit that he was at least subjectively sincere and believed that he was receiving revelations from God.

W. Montgomery Watt states:

His readiness to undergo persecutions for his beliefs, the high moral character of the men who believed in him and looked up to him as leader, and the greatness of his ultimate achievement - all argue his fundamental integrity.  To suppose Muhammad an impostor raises more problems than it solves.  Moreover, none of the great figures of history is so poorly appreciated in the West as Muhammad. (W. Montgomery Watt, Mohammad at Mecca, Oxford 1953, p. 52)

Sir William Muir said:

It is strongly corroborative of Mahomet's sincerity that the earliest converts to Islam were not only of upright character, but his own bosom friends and people of his household; who, intimately acquainted with his private life, could not fail otherwise to have detected those discrepancies which ever more or less exist between the professions of the hypocritical deceiver abroad and his actions at home. (Sir William Muir, The Life of Mahomet, page 54)

And:

I agree with Sprenger in considering 'the faith of Abu Bakr the greatest guarantee of the sincerity of Mohammedin the beginning of his career' - and, indeed, in a modified sense, throughout his life. (Ibid., page 56)

J.W.H. Stobart said:

Abu Bakr was a man of the purest character. His friendship for Mahomet, and unwavering belief in his mission, are a strong testimony to the sincerity of the prophet. (J.W.H. Stobart, Islam and its Founder, page 209)

Tor Andrae said:

The genuineness and sincerity of Mohammed's piety, and the honesty of his belief in his religious call, are indisputable. (Tor Andrae, Mohammed: The Man and his Faith, page 185)

John Gilchrist said:

We can safely reject the view that Muhammad was a deliberate impostor. Throughout the twenty-three year period of his assumed ministry, he held to the unflinching conviction that he was called to be a prophet and that the revelations he was receiving were coming to him from above. (John Gilchrist, Muhammad and The Religion of Islam, Chapter: A Study of Muhammad's Personality: An Assessment of His Personality)

Arthur Glyn Leonard said:

If ever a man on this earth found God, if ever a man devoted his life to God's service with a good and great motive, it is certain that the Prophet of Arabia (Muhammad) is the man. Muhammad was not only the greatest but truest man that humanity has ever produced. (Arthur Glyn Leonard, Islam, her moral and spiritual value: A Rational and Psychological Study, pages 18-19)

Here we see that even non-Muslim critics of Muhammad (peace be upon him) had to at least admit that he was sincere and believed he was receiving revelation from God. In that case, it is difficult to imagine that the Prophet (peace be upon him) knowingly plagiarized material and included it into the Qur'an.