Things that can keep a furnace from lighting, Note, skip to 4, then skip 5
1: Bad thermostat (yours is good, or it would not spark)
2: Bad sail switch (Yours is most likely good, see above)
3: Bad mother board (See above)
4: Bad gas valve (Solenoid), 4B: Gas turned off 4C Bad regulator
5: Improper igniter adjustment (But you see it sparking so not this is low on the list)
6: (PRIME SUSPECT) Clogged burner or orifice Take the burner assembly apart CAREFULLY and clean it out
To check mother board, In fact to check the first 4 items measure voltage at the solenoid coil, 12 volt good, zero volt bad
5 You saw it spark
4 is a bit tricker