Luke doesn’t write, “Day by day the Lord added to their number those who were certainly going to heaven.” Salvation has a number of meanings in the Bible, only one of which refers to going to heaven.
Sometimes it means bodily healing, as when Jesus says to the blind beggar, “Have sight; your faith has saved you” (Lk 18:42). Christ himself was “saved” in this sense when he was raised from the dead, as all Christians will be at the resurrection.
In addition to salvation of the body, there’s the salvation of the spirit. Spiritual salvation comes in various forms. There’s having your sins forgiven and being embued with the sanctifying presence of the Holy Spirit. This is the sense in which the believers in Acts 2:47 were being saved–they were being saved from sin.
There’s also the salvation we “work out” through the power and impulse of God at work in us (Phil 2:12-14). This is the process of sanctification or growth in the life of holiness and righteousness. Then there’s the completion of this process which must be occur for us to enter heaven (Heb 12:14).
Only salvation of the last kind guarantees heaven. All the other forms of spiritual salvation can be lost or undone through serious (mortal) sin.