1John 5:1

Is that Legal?

Greek Πᾶς ὁ πιστεύων ὅτι ἐστιν Ἰησοῦς ἐστιν ὁ Χριστός

It is legal, and it is a relatively standard order in Koine Greek. But specifically with the verb "to be" ?

ὁ ἄνθρωπος Γαλιλαῖός ἐστιν. Luke 23:6

"the man a Galilean he is."

The sentence in Luke 23:6 is translated as "The man is a Galilean." Greek word order is flexible with the verb often appearing after the subject and adjective. Subject + Predicate Adjective + Verb.

Ἔρημός ἐστιν ὁ τόπος Mark 6:35

"Desolate is the Place"

This is written "backwards" from how it would be translated into English, "The Place is Desolate" 

In the case of subject nouns, a sentence like ὁ ἄνθρωπος ὁ βασιλεὺς ἐστιν ("The man the king he is"), ὁ βασιλεὺς ("the king") is the second noun (which is also in the nominative case and refers to the same subject as ὁ ἄνθρωπος), and the verb ἐστιν follows after the second noun. The translation, "the man is the king." 

Βασιλεὺς Ἰσραήλ ἐστιν. Matthew 27:42

A king of Israel he is. 

ὥστε κύριός ἐστιν ὁ Υἱὸς τοῦ ἀνθρώπου... Mark 2:28

"so a lord he is, the Son of the Man..."

All the translations translate the order of these words in reverse, "So the Son of Man is Lord..."