It is difficult to decide whether this fact is enough to trust a document or not. On the one hand, the fact that Lincoln wrote this document himself suggests that he most likely agrees with the ideas expressed in it. Moreover, it also implies that the document was drafted at the time the events described were taking place, making it a primary source. On the other hand, the simple fact that Lincoln wrote the document does not mean that it is accurate. However, overall, the fact that the document is a personal letter makes me more likely to trust it.