Is having a lustful thought a sin?

The holy fathers tell us that if we have a lustful thought, it is just a lustful thought. That is it. There's no sin in it. It could be an image, somebody we see, something immodest on the internet. It is simply a provocation for lust, the first stage. Lust is going to be with us as a temptation throughout the duration of our life, especially if you're a man, because of the way we are physiologically built.

But that does not mean we have to let it move from our head down into our heart where we begin to embrace it. When we embrace the enticement, that is when it becomes a sin. When we begin to think about how to fulfill the desire, or we act on it, we are in full-blown sin.