The goal is to display an icon and text inside of a submit button.
<%= f.submit "Post", class: "new_post_submit" do %>
<i class="glyphicon glyphicon-pencil"></i>
<% end %>
<%= f.submit class: "new_post_submit" do %>
<span><i class="glyphicon glyphicon-pencil"></i> Post</span>
<% end %>
Try this
<%= button_tag(type: "submit", class: "new_post_submit") do %>
<span><i class="glyphicon glyphicon-pencil"></i> Post</span>
<% end %>
This generates html similar to
<button type="submit" class="new_post_submit">
<span><i class="glyphicon glyphicon-plus"></i> Post</span>
</button>
Let me know if this works.