First, the Church’s full doctrine of the Trinity would not be developed until 381 at the Council of Constantinople, at which the Church defined the Divinity of the Holy Spirit.
Second, just because a doctrine is defined at a Council does not mean it is from man. Remember Christ promised the apostles the guidance of the Holy Spirit in Jn 14:26 and invested them with heavenly authority in Matthew 18:18. So, when the Church definitively teaches something on matters of faith and morals, then it carries Divine Authority.
Third, although the Trinity as has been defined by the Church is not in scripture (e.g., 3 co-eternal persons possessing 1 Divine Nature), the elements that make up the definition are. Along with the Father, the bible reveals Jesus and the Holy Spirit is God.
There are scores of examples but I’ll just provide one example for each. In John 1:1-3, we read that the Word, Jesus, was God and that everything that was created was created through him. Concerning the Holy Spirit, in Matthew 12:31 Jesus speaks of blasphemy against the Holy Spirit. If blasphemy is a sin against God, and Jesus speaks of blasphemy against the Holy Spirit, then the Holy Spirit must be God.
From the biblical data, we’re able to conclude that the three Divine persons are distinct, but yet they are one in nature since they are all referred to as God.
Here is a tract that may be of some help:
catholic.com/tracts/the-trinity