If changing all user input "<" to &lt; before rendering in HTML, can I say it's completely safe now?

Nope, it may be very easy to exploit depending on where you render the input. See http://ha.ckers.org/xss.html

No. You have to consider that the attacker can be use something like that to inject javascript when some data of a get or a post data is used to set a url.
e.g. <a href="http://sample.com/?var=(RECEIVE DATA)">
If the attacker inject something like that: " onmouseover="alert(1)" bad=,
he can run javascript on your website.