the first impression was

My first impression of the organization was that it was not going to make enough of a difference with the plans I had in mind.

我对这个组织的第一印象是，它不会对我心中的计划产生足够大的影响。

Although you know your first impression was wrong, your gut response to your new colleague will not be influenced by the first impression only in contexts that are similar to the party.

尽管你知道自己的第一印象错了，但是只有在与派对情境相似的情况下，你对此人的本能反应才会不被原有印象所左右。

My first impression of the explosion was the very intense flash of light, and a sensation of heat on the parts of my body that were exposed.

我的第一印象是爆炸形成了非常强烈的闪光，身体的暴露部分感到一股热浪袭来。