Oida. It refers to their inward acquaintance with their qualities, not objective knowledge.
'Devoted themselves,' &c. In sum this is the sense; but the force of the apostle's phrase is not wholly given. The Greek word means 'appointed to,' as an officer to a regiment. The family of Stephanas had appointed themselves to the saints for service -- given themselves up to serve them, or rather given themselves up to them. 'The saints' is governed by the verb, not by 'service.' The idea resulting from what I have given in the text is more what the apostle means.