He also failed to speak clearly on the issue of what day to worship the Lord. Yet no mainline Protestant will deny that it's God's will that we worship Him on Sunday instead of Saturday.
There are two things to remember:
1) God chose to bring us salvation a) as a human, b) constrained to human time, and c)vulnerable to human suffering.
2) Christ chose fallible, time-constrained human beings to spread His Gospel far and wide, instead of leaving His disciples a book with "all" the answers and "all" topics covered. If all the answers were apparent and obvious, the Church Fathers were wasting their time having an opinion on anything. The belief in Christ's divinity wasn't decided overnight, nor was the hypostatic union. In God's good time, these beliefs were defined. The same goes for the Blessed Mother's Immmaculate Conception and Assumption. There's no way to point to spot on the timeline and say, "This is where the Church should have defined all it needed to define." That would be ludicrous.
For example, there was a space of time where the Church, was in conflict over continuing to heed the requirement of circumcision. Since Jesus did not (according to the Gospels) define the need (or lack thereof) for circumcision, we must then decide if Paul was simply innovating (since there was no apparent admonition from Christ) or if he was properly applying the explicit teachings of Christ to human reason, thereby coming to the conclusion that since the Old Covenant no longer applied (but was fulfilled), thus circumcision was no longer necessary, nor were the dietary laws of Moses. (It should be noted that he who defined it to the entire Church was not Paul, but Peter, who was advised on the matter by Paul, and verified directly by the Holy Spirit.)
Thus, going back to the very first years of Christianity, the Church was formulating doctrine not just on the words of Christ, but as logical extensions of His teachings, thereof. Because it took many many centuries to formally define the dogmatic nature of the Immaculate Conception and the Assumption, that has no bearing on whether the Church has the right or the authority to formulate doctrine which may not be explicitly stated in the Bible. If the Christian Church can accept that because Christ was Resurrected on Sunday, that should be the Lord's Day, then why can't the Christian Church accept that because Christ was sinless, He could not have been begotten through sinful flesh?