The Perennial Philosophy

Jesus shook Buddha's hand and called him brother.

Blake pretty generally adherred to the Perennial Philosophy, which has existed since time immemorial;
it's not a denomination; it's universal.  It has
equal relevance to 'Turk and Jew' (by which Blake
meant everybody; in his day 'Turk' meant Muslims
or Asians-- or simply non-Westerners.)

All of which means that exponents of the perennial
philosophy were not 'Christians', but worldlings
(not in the sense of Paul's 'bad world', but
denoting those with a universal outlook).  Buddha
might have equal weight with Christ.

Among Blake's special friends were Shakespeare,
Spenser, Behmen (Boehme), Trismegistus, Plato,
Thomas Taylor, etc., all who held a universal
outlook. 

There are those who believe that during the period
of his life -- aged 12-30 -- Jesus may have gone to Asia
to visit with and learn from Buddhists and others.
Some believe (or profess to believe) that he may have gone with his Uncle Joseph to Cornwall and established the English church:

"     And did those feet in ancient time,
     Walk upon Englands mountains green:
     And was the holy Lamb of God,
     On Englands pleasant pastures seen!"

There are many definitions of the word
'Christian', but I like best the one that Blake
gave to us in his inscriptions on the Lacoon
(Erdman 273-5):

"The whole Business of Man Is The Arts & All Things Common......Christianity is Art & not Money
Money is its Curse.....
The Old & New Testaments are the Great Code of
Art ....Jesus & his Apostles & Disciples were all
Artists....ART is the Tree of LIFE GOD is JESUS
....Art Degraded Imagination Denied War Governed the Nations

Prayer is the Study of Art
Praise is the Practise of Art
Fasting &c. all relate to Art
The outward Ceremony is Antichrist

A Poet a Painter a Musician an Architect: the Man
Or Woman who is not one of these is not a
Christian"

(Blake has obviously defined the word 'Art' in a
very special sense, but it describes his life and
explains the enormous contribution which he made
to the Christian culture.)