In them he has abundantly demonstrated how his mind was engaged with Scripture, his heart always deeply stirred by divine matters, and his spirit transported from idle preoccupation with the world to contemplation of heaven. In summary, what a steadfast and experienced practitioner of his profession he was, so that he may rightly be regarded as a second Thomas à Kempis, albeit a Reformed one, in our age.
Willem Frijhoff and Marijke Spies, Dutch Culture in a European Perspective: 1650, Hard-Won Unity, pp. 363-364:
This was no small praise, considering that Voetius called the latter's Imitation of Christ a godly book second only to the Bible. A Reformed Thomas à Kempis -- this was indeed important, for even though De imitatione Christi was read by many Protestants it was still considered a Roman Catholic book. Willem Teellinck had in fact been apprehensive of its Catholic qualities. He had used the work extensively and quoted entire passages from it, but without citing the source and always within the framework of his own Calvinist doctrine.
Voetius solved this problem differently. He was able to do so thanks to a new translation of the work with a clearly Protestant slant. The translator, the jurist Cornelis Boey (Boyus) ... had added poems with a Calvinist tenor and left out the entire fourth part -- which dealt with the Holy Communion and was thus unacceptable to Protestants. When this translation came off the press, Voetius wrote a recommendation, stressing the importance of the work for the "inner exercise of godliness and devotion." He also wrote an introduction in which he cautioned against certain Catholic elements one might encounter in the text; and he suggested a certain reading "order."
Both Teellinck and Voetius saw the great value of à Kempis' stress on the devotion of the inner spiritual life to God, and desired to extract that and present it to the world in a Reformed context for edification. This is particularly evident in Teellinck's Sleutel der Devotie openende de Deure des Hemels voor ons (The Key of Devotion Opening the Door of Heaven), in which Teellinck borrowed much from à Kempis, without explicit attribution, with the aim of setting his extracts squarely within a Reformed Protestant context.
Arie de Reuver, Sweet Communion: Trajectories of Spirituality from the Middle Ages through the Further Reformation, p. 113:
Teellinck's ambivalent assessment, however, does not detract in the least from the fact that countless times he quotes from Thomas' Imitation. Striking examples of this are, first that his main source consists of a Roman Catholic translation of à Kempis' well-known tract, and second that in his Sleutel Teellinck not only quotes from its first three books, but cites copious passages from the fourth book, which deals with the eucharist, the section that in the Reformed editions of the day was intentionally omitted. To be sure, not just the Sleutel contains a great deal of à Kempis material. The same holds true for his Soliloquium and for his Het Nieuwe Ierusalem. One hardly needs to guess at the cause of Teellinck's "ecumenical" endeavor. The cause is that he recognized himself in a mystical spirituality borne by experiential communion with God and a glowing love for Jesus. While he did not hesitate to cleanse this pre-Reformation spirituality of its "impure stains," his perspective nevertheless embraced a breadth that crossed its own confessional boundaries. The purpose would have been none other than an evangelistic one. For Teellinck was certainly Reformed to the core, but at the same time he was also Christian to the core. As a Reformed Christian he accorded true devotion not just to the Reformed people, but to the entire Christian tradition. He wanted to make this catholic breadth recognizable expressly by his ties to late medieval spirituality.