It was called 'strange' for it was not authorized by God.
Lev 10:1 "And Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the LORD, which he commanded them not."
Where has God authorized (Col 3:17) the NT Christian to use IM in worship? Or to celebrate Easter/Christmas as religious holidays?
"Which he had not commanded them ..." This is the true key to understanding the sin of Nadab and Abihu. Whatever they did here, it was totally upon their own PRESUMPTUOUS initiative, unsupported by any word whatever from the Lord. The many speculations about what their sin actually was are idle. All that they did here was SINFUL. Their taking of censers, unbidden, their putting incense upon censers carried by themselves, instead of sprinkling it upon the proper altar, their intrusion into the sanctuary in the circumstances and at the time of these events, their taking coals of fire from some place other than from the brazen altar where God had commanded the coals to be taken - all of these things were exceedingly sinful. Why? God had not authorized or commanded a single one of the things that they did.
Can people today commit this sin? Of course, it is impossible for people to commit exactly this sin in the form it appears here, but we must agree with Kellogg that, "As regards the inner nature and essence of this sin, no sin in all the ages has been more common."[1] What about the countless innovations and variations of Christian worship today? How many things there are which so-called Christian churches are doing "as worship of God," which are absolutely nothing else than the teachings and doctrines and practices invented by men and imposed upon the true worship! The frightful example of these unfortunate sons of Aaron serves as a grim warning in such matters."
Coffman Comm. (my emp)