Was the sacrament of penance always present? If so, why did Constantine and other people of his time withhold baptism until they died?
John, chapter 20. Jesus came to His chosen apostles of His Church after His resurrection
"22... he breathed on them and said: Receive the Holy Spirit.
23 If you forgive anyone's sins, they are forgiven; if you retain anyone's sins, they are retained."
The Catholic Encyclopedia, Confession may answer some or all of your questions regarding Confession
This should be of interest.
Confession has been present from the early days of the Christian faith, but for some time was performed in public and/or could only be partaken of once in a Christian’s life. Baptism was delayed until death so Constantine, along with those who thought like him and likewise wished to live as they pleased, could avoid the heavy penances that would come following the sacrament of confession.
Actually, public penance and confession occurred long before Christ. Repentance and confession are shown in Nehemiah 9:1-3, which detail historical events in Israel after their return from the Babylonian exile (from about 538 BC to 410 BC*)
- Inside the Bible by Fr. Kenneth Baker, SJ