The statement is clear in its sense of the superior status of preserving the Union. It is obvious to see that Lincoln was determined that his actions upon emancipation should depend on whether it would benefit the Union. And if it would not help saving the Union, he would choose to not do anything about freeing the slaves. He expressed similar sentiments in his First Inaugural Address in 1861 March. Facing the possibility of more states seceding and more splits emerging between the North and the South, he tried to eliminate the concerns of slaveholders by clarifying that he had no inclination to free the slaves. His remarks were so extreme, almost proslavery that it is easy to see how eager he wanted the slaveholders to be less worried, and the Union to be as united as possible. In both examples, for Lincoln ending slavery was an inferior object compared to saving the Union. It was understandable because Lincoln was the president of the United States and preserving the Union was part of his responsibility. However, it also meant that he would not hesitate to stop the emancipation process once the Union's interests were under threat.
Besides the oneness of the Union, Lincoln had another matter of significance on his mind that accounted for his delaying the emancipation: the Constitution. He mentioned in various speeches and public as well as private letters of his that holding slaves, as a lawful right could not be interfered. One example is a private letter he wrote to a close friend Joshua Speed. As the letter was private, I believe that the content was in closest proximity to Lincoln's genuine feelings. Lincoln first talked about how he disliked slavery because of its wrongness in multiple aspects. "But", he said, "I am not aware that any one is bidding you to yield that right; very certainly I am not I also acknowledge your rights and my obligations, under the constitution, in regard to your slaves" (Lincoln, 1).
