Learn English – a one-word synonym for “religious symbol”

I have some suggestions. The first, 'hierogram', is not a neologism, and on the whole strikes me as the best option:

hierogram, n.
A sacred symbol; ... (lit. and fig.).

["hierogram, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/86816?redirectedFrom=hierogram (accessed March 04, 2016).]

The OED Online definition matches the definition found in Collins English Dictionary.

hierogram (ˈhaɪərəˌɡræm)
n
a sacred symbol

[hierogram. (n.d.) Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. (1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014). Retrieved March 4 2016 from http://www.thefreedictionary.com/hierogram ]

My other suggestions, 'Tetragrammaton' and 'symbol', do not seem as suited to the question. 'Tetragrammaton' was sometimes used with the general figurative sense of

†c. fig. An emblem or symbol of something sacred. Obs. rare.

["Tetragrammaton, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/199850?redirectedFrom=tetragrammaton (accessed March 05, 2016).]

This word is only considerable because the question specified that a neologism might be acceptable. Reviving the obsolete and rare figurative use of 'Tetragrammaton' would be neologistic and, additionally, might risk confusion with the extant sense, which is much more detailed and specific, and so not particularly suitable as an answer for the question:

a. A word of four letters; spec. the Hebrew word written yhwh or jhvh (vocalized as ya hwe h, ja hve h, or je ho va h, q.v.); often substituted for that word (regarded as ineffable), and treated as a mysterious symbol of the name of God; sometimes used as a title of the Deity (see quot. 1689).

(op. cit.)

The other suggestion, 'symbol', does not seem to answer the question, for obvious reasons (the request was for a 'religous symbol', and 'symbol' is currently used with a much broader denotation), although the earliest uses of 'symbol' were specific to 'religious symbols':

1. a. A formal authoritative statement or summary of the religious belief of the Christian church, or of a particular church or sect; a creed or confession of faith, spec. the Apostles' Creed.

This use is traceable to Cyprian, Bishop of Carthage (c250), who applies Latin symbolum to the baptismal creed, this creed being the ‘mark’ or ‘sign’ of a Christian as distinguished from a heathen. The notion, long current, that the creed was so called because it was ‘put together’ by the Apostles is without foundation in fact.

["symbol, n.1". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/196197?rskey=1gOSeD&result=1&isAdvanced=false (accessed March 05, 2016).]

Disambiguation: 'hierogram', 'hieroglyph', 'hierograph'

Some confusion about the uses of 'hierogram', 'hieroglyph' and 'hierograph' became evident in the comments on this answer. Of the three, 'hierogram' is the only one used with the general and primary sense of 'a sacred symbol' (as shown in the definitions given above). 'Hierogram' is sometimes used, in technical contexts, to mean 'hieroglyph', but that is a secondary sense confined to specialized uses with reference to, specifically, a subset of hieroglyphs regarded as sacred.

'Hieroglyph', in contrast, refers to

1. A picture or symbol used in hieroglyphic writing.
2. Something that suggests a hieroglyph.

[hieroglyph. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved March 5 2016 from http://www.thefreedictionary.com/hieroglyph ]

'Hieroglyph' does not denote sacredness, and has no necessary connotation of sacredness.

'Hierograph', on the other hand, does denote sacredness, but is used specifically to denote sacred writing, characters, or inscriptions:

hierograph (ˈhaɪərəˌɡrɑːf) n
sacred writing or characters.

[Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. S.v. "hierograph." Retrieved March 5 2016 from http://www.thefreedictionary.com/hierograph ]

Another definition, from OED Online, where the close relationship between 'hierographs' and 'hieroglyphs' is made plain,

hierograph, n
A sacred inscription or symbol: a hieroglyph.

["hierograph, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/86819?redirectedFrom=hierograph (accessed March 05, 2016).]