It seems that the second of the ten commandment would exclude "any shape" whatsoever to be used in worship.

(Exodus 20:4) “You must not make for yourself a carved image or a form like anything that is in the heavens above or that is on the earth underneath or that is in the waters under the earth.

Was there an exception made concerning the cross, or was the second command somehow altered or canceled ? If not, can the use of the cross in worship be considered idolatry ?