In earnest
Definition 1 of 2
Pro Tip 1/3Emphasizing Dedication
Use it to stress the intensity and commitment involved in an action or decision. 
He approached his studies in earnest, knowing his future depended on it.
Pro Tip 2/3Not Light-Hearted
Suggests a shift from a casual attitude to a more serious one. 
After the practice rounds, the competition started in earnest.
Pro Tip 3/3Conveys Sincerity
Indicates genuineness, opposite of pretending or joking. 
When he apologized in earnest, I knew he meant it.