A half truth presented as a whole truth is a complete untruth.
"All you need to know about Jesus's death is that He died on a cross." (This leaves out most of the who, what, where, when, why, and how of the crucifixion.)

A half truth presented as a half truth is a whole truth.
"Part of what the Bible teaches about Jesus's death is that He died on a cross." (True.)

A whole truth presented as a half truth is a complete untruth.
"Jesus is the Way to Heaven, but there are other ways to Heaven as well." ("Jesus is the Way to Heaven" is the whole truth. But if He is presented as one of many ways to get to Heaven then the whole presentation is a complete untruth- even though it contains a whole truth.)

A whole truth presented as a whole truth is a whole truth.
"Jesus is the only Way to Heaven." (True.)