The answer to this is that when we think of Jesus Christ and we think that he was a great model for morality, we only think so because we compare his actions to an ideal that we have in our minds of what moral actions look like. We are not really using Jesus as the model. Instead, we are comparing Jesus to a mental model. This means that we are really just using him as something of an example of the real thing. As Kant says:
… every example of it (morality) that is set before me must be first itself tested by principles of morality, whether it is worthy to serve as an original example, i.e., as a pattern; but by no means can it authoritatively furnish the conception of morality. Even the Holy One of the Gospels must first be compared with our ideal of moral perfection before we can recognise Him as such…
This is not to say that we should not think that Jesus was morally good. But it is to say that we should not use Jesus as our model because we only know if he was morally good by comparing him to our ideas of what is morally good.
This is part of Kant’s argument that all imperatives are things that we must know a priori. If we rely on examples as the starting points for our beliefs, we will go astray. One reason for this is that we never actually know why a person does a given thing. We might think that they are doing this thing with a good will when they actually have some ulterior motive that is less than good.
To Kant, the moral law is something that clearly exists for us a priori. He says that this is obvious because we have an innate sense of morality. We all think in moral terms all the time. This implies to him that there must be some sort of a priori moral law. If the moral law does exist a priori, we cannot derive it from examples. It has to be a pure law and examples are never pure. Examples depend on circumstances and may not apply in all times and places. Something like morality must apply in all times and places.
So, moral law exists a priori. Something that exists a priori cannot be derived from examples. Jesus would be an example and therefore we cannot derive our moral law from looking at him. We must, instead, have a concept of morality already in place and compare Jesus (and our own actions) to that.