With this combination of pastoral work and college life Mackenzie was perfectly satisfied and happy, but in another year the turning-point of his life was reached. A mission at Delhi to the natives was in prospect, and the Rev. J. S. Jackson, who belonged to the same college with him, came to Cambridge in search of a fellow-labourer therein. During the conversations and consultations as to who could be asked, the thought came upon Mackenzie, why should he strive to send forth others without going himself. He could not put it from his mind. He read Henry Martyn's life, and resolved on praying for guidance as to his own duty. In the words of his letter to Mrs. Dundas, "I thought chiefly of the command, 'Go ye and baptize all nations,' and how some one ought to go; and I thought how in another world one would look back and rejoice at having seized this opportunity of taking the good news of the Gospel to those who had never heard of it; but for whom, as well as for us, Christ died. I thought of the Saviour sitting in heaven, and looking down upon this world, and seeing us, who have heard the news, selfishly keeping it to ourselves, and only one or two, or eight or ten, going out in the year to preach to His other sheep, who must be brought, that there may be one fold and one Shepherd; and I thought that if other men would go abroad, then I might stay at home, but as no one, or so few, would go out, then it was the duty of every one that could go to go. . . . And I thought, what right have I to say to young men here, 'You had better go out to India,' when I am hugging myself in my comfortable place at home." And afterwards, "Now, dear Lizzie, I have always looked to you as my mother and early teacher. To you I owe more than I can ever repay, more than I can well tell. I do hope you will pray for me and give me your advice." Mrs. Dundas's reply to this letter was a most wise and full expression of sympathy with the aspiration, given with the deep consideration of a peculiarly calm and devotional spirit, which perceived that it is far better for a man to work up to his fullest perception of right, and highest aims, than to linger in a sphere which does not occupy his fullest soul and highest self; and she also recognized the influence that the fact of one of a family being engaged in such work exercises on those connected with them. Others of the family, however, were startled, and some of his Cambridge friends did not think him adapted to the Delhi Mission, and this therefore was given up, but without altering the bent that his mind had received; and indeed Mrs. Dundas, in one of her beautiful letters, advised him to keep the aim once set before him in view, and thus his interest became more and more turned towards the support of missionary work at home.