Sincere friendship towards God, in all who believe him to be properly an intelligent, willing being, does most apparently, directly, and strongly incline to prayer; and it no less disposes the heart strongly to desire to have our infinitely glorious.

This quote is saying that when someone has a sincere and strong belief in God, then it encourages them to pray and to desire for their relationship with God to be even stronger and more meaningful. In other words, if someone trusts and believes in God with all their heart, it will motivate them to pray and to strive for a better connection with God.