Radak: His great strength, that He redeemed them from the affliction that they were in via His great strength and strong hand. Malbim - on the same day that He redeemed them, they forgot His strength! Immediately after Ge'ulas Mitzrayim, they forgot His wonders and thought that He cannot do more miracles - "va'Yamru Al Yam b'Yam Suf" (Tehilim 106:7).