"secular" basically means worldly--anything that isn't spiritually or religiously inclined. examples of "secularism" might be self-indulgence of alcohol or erotic material, or just enjoying certain human pleasures that have no religious reward, such as gourmet food.
humanism basically put humans above all else. they set aside the biblical stories that had religious value and started focusing on more mundane issues. they pointed out the goodness and usefulness of people rather than relying on religion. in this way, humanism promoted secularism because it was promoting humans themselves. it turned away from the strong traditional customs and encouraged humans to problem-solve on their own. it encouraged less reliance on religion and highlighted the abilities of people.
as a disclaimer, i'd like to add that this is just historical context i've gathered from my art history courses. further research might grant you more thorough observations!!