The dictionnary says this in French:

Action d’élever. L’érection d’une statue, d’une chapelle.

The noun is derived from ériger (to erect) and has turned to erection

The act of building or putting up or together of something; construction.

So basically, your sentence says the church was build in date of the 16th april 1450.

Here acte is a legal term, it means that there is a legal document/deed bearing that date (16 avril 1450) and the signature of the rightful authority, granting permission to build a church on that particular ground.
The proper building up of the church could have started months or years after that date, or never have occurred.

Here, “l’acte […] porte la date […]” implies that the date is somehow on the acte. It’s like “the act (…) bears the date (…)” in English. So acte has to be in the sense of an official document, rather than in the sense of the physical action.

“L’acte d’érection” on its own could go either way, but it would be an odd way to say that the action took place on that date. A more idiomatic formulation would be “Le bâtiment a été érigé le 16 avril 1450”. It would be odd that a building had been erected in one day; this could make sense for something like a mast.

Actually, in this context, I’m not sure whether érection refers to the church as a building or to the church as a congregation. The expression “acte d’érection” is used for both kinds.

Either way, from a linguistic point of view, there is too little information in your question to conclude what the date is. It could refer to the date the action took place (whether it’s the date the first stone was laid, or the date the building was completed, or the date the congregation started to exist officially), or to the date the act was written or signed (and I don’t even know whether that would be an act taking a decision or an act stating that an action had been performed).

From a historical perspective, I think “acte d’érection” refers to the decision to found the community (agreeing with Laure). Example:

Fondation de la paroisse Saint-Marc: 22 mars 1792

Cette permission de construire d’abord un presbytère dans le haut duquel on fera le Service divin est accompagnée de l’acte d’érection d’une paroisse en la seigneurie de Cournoyer, désignée sous le nom de Paroisse de Saint-Marc l’évangéliste.

L’inauguration et la bénédiction de la nouvelle chapelle, érigée en haut du presbytère, se déroulent sous la présidence du vicaire général Denaut, le 17 décembre 1793. Le curé désigné est Joseph Martel, curé de la paroisse de Saint-Charles-sur-Richelieu.

(Summarizing: the parish was legally created on 1792-03-22, by an “acte d’érection”. The church itself was inaugurated (i.e. ceremonially first used) a year and a half later.)