To be earnest, to keep going forward with whatever you have decided to do, and to do it with a pure heart.
この場合、次のような言い方ができますよ。
ーTo be earnest, to keep going forward with whatever you have decided to do, and do it with a pure heart.
「真面目に、真っ直ぐ、そして真っ白に」
earnest「真面目な」
to keep going forward with whatever you have decided to do「やると決め前に進み続ける」
a pure heart「けがれのない心」
ご参考まで!