Am I a soldier of the Cross? I. Watts. [Holy Fortitude.] Appended to his Sermons, published in 1721-24, in 3 vols., vol. iii., and intended to accompany a sermon on 1 Cor. xvi. 13. It is in 6 stanzas of 4 lines, and entitled "Holy Fortitude." In Spurgeon's Our Own Hymn Book, No. 671, st. v. and vi. are omitted, but the rest are unaltered. Original full text in all editions of Watts's Works. In the New Congregational Hymn Book, No. 623, it is given in an abbreviated and slightly altered form as ā "Are we the soldiers of the Cross?" This is also found in Snepp's Songs of Grace & Glory, 1872, and other collections. It dates as early as the Leeds Hymn Book, 1853. The American use of this hymn is extensive.
-- John Julian, Dictionary of Hymnology (1907)
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Am I a soldier of the Cross? p. 55, ii. In some annotations of this hymn we have found it stated that it is a portion of "Do I believe what Jesus saith?" that the hymn in its full form appeared in Watts's Hymns & Sacred Songs, 1709; and that the portion beginning "Am I a soldier of the Cross?" subsequently appeared in his Sermons, 1721-24. The facts are (1) both hymns appeared in the Sermons, &c, 1721-24, āDo I believe, &c," for Ser. 30, and "Am I a soldier, &c." for Ser. 31; and (2) the two were united as one hymn, and included in the Hymns and Sacred Songs, Bk. i., No. 37, Pt. ii. sometime in the present century. We find it in T. Nelson's edition of Watts, 1844, and others, but in no edition before 1800.
--John Julian, Dictionary of Hymnology, Appendix, Part II (1907)