The following are quotes by Samuel Zwemer on the importance of the Scriptures and Trinity in witness to Muslims.
From The Moslem Doctrine Of God.
The Mohammedan controversy with Christians has ever had two great centres; and although the form of the ellipse has changed since the days of Raymund Lull, or even since the time of Henry Martyn, the foci remain the same. The integrity of Scripture and the reasonableness of the doctrine of the Trinity are the two points in Christianity against which Islam emphatically testifies. At the same time these two ideas are fundamental in the Christian system. The doctrine of the Trinity is not only fundamental but essential to the very existence of Christianity. Dr. Baur of the Tubingen school acknowledges this when he says that “in the battle between Arius and Athanasius the existence of Christianity was at stake.” In some form the doctrine of the Trinity has always been confessed by the Church and all who opposed it were thrown off from its fellowship. “When this doctrine was abandoned, other articles of faith, such as the atonement, regeneration, etc., have almost always followed, by logical necessity, as when one draws the wire from a necklace of gems, the gems all fall asunder.” (Henry B. Smith.) The doctrine of the Trinity, in its widest sense, includes that of the Incarnation and of the Holy Spirit. (p. 78)
Islam is proud to write on its banner, the Unity of God; but it is, after all, a banner to the Unknown God. Christianity enters every land under the standard of the Holy Trinity-the Godhead of Revelation. These two banners represent two armies. There is no peace between them. No parliament of religions can reconcile such fundamental and deep rooted differences. We must conquer or be vanquished. In its origin, history, present attitude and by the very first article of its brief creed, Islam is anti-Christian. But that does not mean that the battle is hopeless. Christian monotheism is as superior to Mohammedan monotheism as Christ is superior to Mohammed. There is no god but the Godhead. Islam itself is beginning to realize the strength of the Christian idea of God, and our chief prayer for the Moslem world should be that they may know the Only True God and Jesus Christ whom He hath sent. When the great Mohammedan world acknowledges the Fatherhood of God they will also understand the brotherhood of men and the mystery of Calvary. (pp. 119-120)